Custom "Home" button and sequence for SmarAct Encoded Tip Tilt

[aberges-SLAC]

Current Behavior

Due to the peculiar "referencing" sequence involved in "homing" SmarAct encoded tip-tilt stages, it would be very useful to capture this procedure in a single "home" button that will prevent future confusion/errors.

Expected Behavior

We can ideally overwrite the home_forward and home_reverse signals that the SmarAct class inherits and replace it with a single button that does the following:
Axis 1

1. home in the "reverse" direction, towards the end stop
2. step forward in open-loop mode (so that you are away from the mechanical end stop)
3. home_reverse again
   Axis 2
4. home in the "reverse" direction towards the end stop
5. step forward in open-loop mode
6. home_reverse again
7. Repeat steps 1-3 on Axis 1 again.

This could also be similarly executed for calibration sequences

Context / environment

The referencing & calibration procedure can be cumbersome for SmarAct encoded tip-tilt devices. Since the mechanical end stop is the only valid reference specified by the vendor, there is no need for a home_forward and home_reverse in the usual context. This sequence for calibration and home can be substituted instead.

Tyler has something similar cooking for the btms gui which prompted this issue being raised.

[aberges-SLAC]

@slactjohnson Issue to track this nice-to-have