Optional Renishaw encoder for MOFOD

It provides the possibility of unguided exposures at arcsecond precision (but no guiding is possible in RA), which means it can be useful for research like SN patrol, minor planet astro/photometry or any other research that requires short exposures (max 5 min). The time limit is due to the fact that even if RA tracking is perfect, factors like polar alignment and flexure (primarily of the OTA) will introduce errors over 1" with more exposure time.