add docstrings to GPS class attrs

[awordforthat]

Addresses issue #82. Added docstrings and built locally with sphinx to check formatting, etc. Some pylint errors addressed while I'm here.

Open question: unused vars and missing parsers

There are two unused attributes on the GPS class, true_track and mag_track. These come from the GPGTV sentence type (if you really want the gory details, see these docs for more). As of right now, the class does not handle this message type. It used to, but it was removed with a few others in this commit. I believe that this was unintentional or incomplete, since the other removed sentence types did not have their parsers removed. However, sometime between that commit and the date of this PR, the other parsers did get removed.

I'll propose that in this PR, we remove the two lingering variables, and raise a new issue/question around whether the old sentence types should come back (or just find out why they were removed). This is already done, just calling attention to it.

Other notes:

Some attributes have multiple options that need to be explained. The additional pipe characters you see are to force sphinx to preserve newlines so that what appears on Read The Docs is nicely formatted, e.g.:

[awordforthat]

tagging @tannewt for potential review

[awordforthat]

also @tekktrik if you want to take a quick look at this