added 'export_as' for parameters and commands

the export argument of an accessible allows now to specify an other
external name than the attribute used internally.

export=True   (default, use defined name, or prefix with '_' when
               name not in predefined list)
export=False  (do not export)
export=<any string> (special cases only)

Change-Id: I6c6669cd502d9d6fd3aa40091673e5554fd961bd
Reviewed-on: https://forge.frm2.tum.de/review/19664
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>

secop/params.py

@@ -29,6 +29,7 @@ from secop.lib import unset_value
 
 EVENT_ONLY_ON_CHANGED_VALUES = False
 
+
 class CountedObj(object):
     ctr = [0]
     def __init__(self):
@@ -36,6 +37,7 @@ class CountedObj(object):
         cl[0] += 1
         self.ctr = cl[0]
 
+
 class Accessible(CountedObj):
     '''abstract base class for Parameter and Command'''
 
@@ -73,6 +75,7 @@ class Accessible(CountedObj):
                 raise ProgrammingError('can not overrride property name %s' % name)
             cls.valid_properties[name] = external
 
+
 class Parameter(Accessible):
     """storage for Parameter settings + value + qualifiers
 
@@ -219,3 +222,23 @@ class Command(Accessible):
     def for_export(self):
         # used for serialisation only
         return self.exported_properties()
+
+# list of predefined accessibles with their type
+PREDEFINED_ACCESSIBLES = dict(
+    value = Parameter,
+    status = Parameter,
+    target = Parameter,
+    pollinterval = Parameter,
+    ramp = Parameter,
+    user_ramp = Parameter,
+    setpoint = Parameter,
+    time_to_target = Parameter,
+    unit = Parameter, # reserved name
+    loglevel = Parameter, # reserved name
+    mode = Parameter, # reserved name
+    stop = Command,
+    reset = Command,
+    go = Command,
+    abort = Command,
+    shutdown = Command,
+)