Stepper motor: make limit switches configurable

[alexdewar]

While it's not yet entirely clear what will be different for UNIRAS in relation to the limit switches, they will at least almost certainly be in different locations, which will require modifications to the code.

Additionally, @jonemurray has mentioned that they could be used for homing in the case that the optoswitch in the home position fails. In order to do this, we would need to disable their function as limit switches, with the DL command (see the manual).

Seemingly the controller also offers the option of setting limits purely with software, so that may be something we wish to consider.

Note that nothing will break if the motor moves past the limits (either in FINESSE or UNIRAS); the main concern is that the motor isn't left rotating indefinitely if its connection to the software is suddenly terminated.