support RPO sentence

[bomilkar]

XCSoar sends polar coefficients to vario. The sentence we are most interested in deals with the real polar. That is the polar after degradation to account for bugs etc. and after applying the load_factor. So these coefficients are ready to use by STF.
The sentence is described here:
https://www.openvario.org/doku.php?id=projects:series_00:software:nmea
under "Real Polar".

An example:
$POV,C,RPO,0.001558,-0.061671,1.154286*5B

This together with the MacCready value is needed to do STF.

[bomilkar]

I will implement this (and only this) unless someone objects.

[bomilkar]

Addressed in #19

I had to fix one bug in stf.c line 50 in order to succeed testing STF.
Other than that, only minimal changes were applied.

Please continue testing! (Honest) feedback appreciated!