Inconsistency in `focuser.move_by`

[danjampro]

The abstract Focuser.move_by returns the final focuser position:

POCS/src/panoptes/pocs/focuser/focuser.py

Line 176 in e79ac49

return self.move_to(self.position + increment)

Whereas the Birger version returns the change in focuser position:

POCS/src/panoptes/pocs/focuser/birger.py

Line 228 in e79ac49

return moved_by