Added additional REL flag parsing, fixed PVT carr_soln parsing

[m-elias]

Parsing additional RELPOSNED flags, added functions to get PVT & REL flag data, fixed PVT carr_soln_ parsing to return fixed(2), float(1) or none(0).

[flybrianfly]

@m-elias, thanks for the contributions. It looks like rel_pos_gnss_fix_ok_ and rel_pos_diff_soln_ are shadowing the current code's fix_ output (FIX_NONE or FIX_DGNSS). I'm not sure what those add, unless the intent was to pull from the UBX-NAV-RELPOSNED packet instead of PVT. The carrier solution flags in UBX-NAV-RELPOSNED should be the same that we're getting in UBX-NAV-PVT carrSoln bits. Have you been seeing inconsistencies between the PVT carrSoln flags and the RELPOSNED flags?

[m-elias]

I was testing your code to use with the AgOpenGPS project's hardware boards. There we use more of REL's flags and I wanted to compare them to PVT's to verify they're both the same. carrSoln should return 3 values as noted in my comment but I don't think it was (I've forgotten those details now). I was also using it to only parse REL by itself so that's why I added in the extra parsing that PVT already had.