Added scaffolding for velocity mode

Added records to support detection of the current operation mode
(position or velocity), whether one is allowed to change between the two
and a record to actually change between the two. Also added a
doMoveVelocity method which should be implemented by derived drivers if
they support velocity mode.

README.md

@@ -117,11 +117,11 @@ does not matter):
 file "$(SINQDBPATH)"
 {
 pattern
-{ AXIS,      M, DESC,                              EGU, DIR, MRES,  MSGTEXTSIZE, ENABLEMOVWATCHDOG, LIMITSOFFSET, CANSETSPEED, ADAPTPOLL }
-{ 1,    "lin1", "Linear motor doing whatever",      mm, Pos, 0.001,         200,                 1,          1.0,           1,         1 }
-{ 2,    "rot1", "First rotary motor",           degree, Neg, 0.001,         200,                 0,          1.0,           0,         1 }
-{ 3,    "rot2", "Second rotary motor",          degree, Pos, 0.001,         200,                 0,          0.0,           1,         0 }
-{ 5,    "rot3", "Surprise: Third rotary motor", degree, Pos, 0.001,         200,                 1,          2.0,           0,         0 }
+{ AXIS,      M, DESC,                              EGU, DIR, MRES,  MSGTEXTSIZE, ENABLEMOVWATCHDOG, LIMITSOFFSET, CANSETSPEED, ADAPTPOLL, CANSETMODE }
+{ 1,    "lin1", "Linear motor doing whatever",      mm, Pos, 0.001,         200,                 1,          1.0,           1,         1,          0 }
+{ 2,    "rot1", "First rotary motor",           degree, Neg, 0.001,         200,                 0,          1.0,           0,         1,          0 }
+{ 3,    "rot2", "Second rotary motor",          degree, Pos, 0.001,         200,                 0,          0.0,           1,         0,          0 }
+{ 5,    "rot3", "Surprise: Third rotary motor", degree, Pos, 0.001,         200,                 1,          2.0,           0,         0,          1 }
 }
 ```
 The variable `SINQDBPATH` has been set in "mcu1.cmd" before calling `dbLoadTemplate`.
@@ -166,6 +166,9 @@ behaviour so that the affected axis is only polled with the busy / moving poll
 period if it itself is moving. This setting is ignored for "forced fast polls"
 (when the poller is woken up, e.g. after an axis received a move command).
 Defaults to 1.
+- `CANSETMODE`: If set to any value other than 0, the operation mode of the
+motor can be changed. See section [Velocity mode](#velocity-mode).
+
 
 ### Motor record resolution MRES
 
@@ -212,6 +215,29 @@ transferred to (motor_record_pv_name).MRES or to
 `sinqMotor` provides a variety of additional records. See `db/sinqMotor.db` for
 the complete list and the documentation.
 
+### Velocity mode
+
+The motor record was originally designed for motors which operate in _position_
+mode: They are given a certain position, move to that position and then stop
+on their own. Some motors however operate in a continuous _velocity_ mode, where
+they move (usually rotate) with a fixed velocity until told to stop or change
+the velocity. To support this operation mode, `sinqMotor` provides three
+additional records:
+
+- `$(INSTR)$(M):Mode`: This read-only record returns 0 if the motor is in
+position mode and 1 if it is in velocity mode. Defaults to 0.
+- `$(INSTR)$(M):CanSetMode`: If the value of this record is other than zero, the
+motor mode can be changed during operation. Defaults to 0, but can be
+overwritten in the substitution file with `CANSETMODE` or from within the driver
+(e.g. by reading out a parameter from the hardware).
+- `$(INSTR)$(M):SetMode`: This record can be used to switch between operation
+modes if `$(INSTR)$(M):CanSetMode` is not zero. Currently accepted values are
+`0` for position mode and `1` for velocity mode.
+
+When in velocity mode, writing to the `VELO` field of the motor record directly
+triggers the corresponding velocity change if the driver supports it. Writing
+to the `VAL` field does nothing in velocity mode.
+
 ## Developer guide
 
 ### File structure
@@ -251,7 +277,10 @@ This is an empty function which should be overwritten by concrete driver impleme
 - `reset`: This function is called when the `$(INSTR)$(M):Reset` PV from `db/sinqMotor.db` is set. 
 It calls `doReset` and performs some fast polls after `doReset` returns.
 - `doReset`: This is an empty function which should be overwritten by concrete driver implementations.
-- `move`: This function sets the absolute target position in the parameter library and then calls `doMove`.
+- `moveVelocity`: This function checks if the motor is in velocity mode. If that is the case, it calls `doMoveVelocity`.
+- `doMoveVelocity`: This is an empty function which should be overwritten by concrete driver implementations.
+- `move`: This function checks if the motor is in position mode. If that is the
+case, it then sets the absolute target position in the parameter library and then calls `doMove`.
 - `doMove`: This is an empty function which should be overwritten by concrete driver implementations.
 - `home`: This function sets the internal status flags for the homing process and then calls doHome.
 - `doHome`: This is an empty function which should be overwritten by concrete driver implementations.