NTRIP: use pynmeagps for correct gga generation

[richahert]

The MAVproxy NTRIP client does not work with several NTRIP casters, because the GGA messages are sent in an incompatible format. this is described in #1131

This PR solves that problem, by using the same library for gga generation that pygpsclient uses (pynmeagps), which I experienced to be working with all casters.

The ntrip client of pygpsclient works quite well, so I might open an Issue in the future to request that the MAVproxy ntrip could be refactored as a wraper of that NTRIP client. It has more active developers than the built-in NTRIP Client of MAVproxy.

For now, this PR should provide a solution that is tested with skylark and I am confident that it also solves the problem with topcon, but I can't test it atm, because I don't have an active subscription.

[rmackay9]

Hi @richahert,

Could you rebase this on master to remove the merge commits?

[richahert]

Hi @rmackay9 ,
I rebased the branch so the PR has one clean commit.

[rmackay9]

Hi @richahert,

Thanks for that. You might want to also prefix the commit message with "ntrip:" or "NTRIP:" to be consistent with other commits in this repo but again, I'm not a maintainer so this is just a suggestion and I'm not sure how strict Tridge and others are.

If this gets stuck and not merged then it might need to be brought up on one of the dev calls to get merged. The best choice is probably the small EU Dev Call which is later today.