RINEX v2 ionosphere parameters support

[fedosgad]

In RINEX v2 files header records with labels ION ALPHA and ION BETA are allowed. They contain coefficients for ionospheric delay calculations for GPS system (GLONASS, which is also allowed in those RINEX versions, does not broadcast any ionosphere-related information and can be safely ignored). Those fields are checked for by header parser but not handled any further (there are TODO notices).

According to Navipedia and IS-GPS-200D (Figure 20-4), coefficients are used for Klobuchar model calculations. This model is already supported by library. IONOSPHERIC CORR field, which replaced ION ALPHA and ION BETA in RINEX v3, is handled by header parser which presents it as a separate IonMessage. I propose that ION ALPHA and ION BETA should be handled in a similar manner, adding a IonMessage wrapping KbModel to resulting Header.

I am willing to make a PR for this issue. It would be my first experience with Rust though, possibly requiring thorough code review (I have previous coding experience in other languages, so maybe it's not that bad). Current developers/maintainers, would you like me to make such a PR?

[gwbres]

@fedosgad,

first off, thank you very much for inquiring and yes any contribution is welcome.
This topic is important because the current state does not allow precise positioning using V < 3 RINEX (so GPS, Glonass and GAL possibly)

[fedosgad]

Great!
I'll take to it then.

[fedosgad]

Created #239

[gwbres]

Thank you for your contribution, this will be added to V0.16 that I'm publishing this week end