Work with virtualagc devs to move to upstream yaAGC

[ThymoNL]

Our counterpart of virtualagc/virtualagc#622
Work is being blocked by us not being in sync with upstream yet.
Use this as a tracker bug for any effort towards unification.

The sync_agc branch contains the upstream files, it does not currently build.
The fix_agc branch contains fixes to make sync_agc build and to test individual patches. This branch may be synced to sync_agc and Orbiter2016 when desired.