remove stretchy arms

I'm removing the "stretchArm" feature.

Also, I added a simple "calibration" mode for the hydras which can sometimes compensate for poorly placed transceiver orbs.  It works like this:

(1) hold arms straight out at sides, about shoulder high
(2) press and hold BUTTON_FWD on both hydras for about 1 second
(3) let go

The calibration will compute a new offset and a rotation adjustment.  The idea is to make it easy to correct for a shifted hydra orb.  At the moment the calibration doesn't work well when the orb is very misaligned (more than about 30 degrees).  I couldn't figure out exactly why this is the case but it appears that the orb itself has some non-linearities, so it is still a best practice to keep your orb aligned correctly.

This work doesn't close issue #2256 but is partial progress.