Symetrie hexapod: set origin from config + convert units
This is a backport of a feature developed by Olivier Ulrich for BM32 (cf. F-CRG/bm32#5)
- set user and object origins defined in the config at startup
user_origin: 0 0 328.83 0 0 0 object_origin: 0 0 328.83 0 0 0
- convert values to axis unit
- Symetrie hexapods work only with mm and deg, but mrad and microns are more useful units