Fix Prosistel rotator control backend

[mikaelnousiainen]

The Prosistel backend is now a bit broken, as it follows the Prosistel Combi-Track (az+el) protocol, which is slightly different from the Prosistel D (azimuth/elevation only) controllers. The az/el-only controllers also do not accept queries for the unsupported angle, e.g. it's an error to query elevation on an azimuth-only controller.

I contacted Prosistel and they actually have all the protocol documentation available here: https://www.prosistel.it/download/download.htm

The documentation reveals that there are three "product families", namely the "D" series, the PRO series and the Combi-Track (az+el) combos.

Of these three, the D and the PRO series seem to use the same protocol with angle in degrees and A/E identifiers for azimuth and elevation. These rotator controllers support either azimuth or elevation, but not both.

The Combi-Track controllers are combined azimuth+elevation controllers and these represent angles in 1/10ths of degrees and use A/B identifiers for unit A and B (which can be thought of as azimuth and elevation).

I've rewritten the backends for Prosistel allowing user to choose between the az, el or az+er models.

I've tested the D azimuth (model 1701) backend on PST-61D rotator and it seems to work now, the other 2 backends need testing.