Added new feature to set deadband

The field SPDB can now be populated via either the substitutions file or
from inside the driver (using the motorPositionDeadband paramLib entry).

src/sinqController.h

@@ -315,6 +315,7 @@ class HIDDEN sinqController : public asynMotorController {
     int motorAcclFromDriver();
     int motorHighLimitFromDriver();
     int motorLowLimitFromDriver();
+    int motorPositionDeadband();
     int adaptivePolling();
     int encoderType();