feature: add MTA, MWD and VWT sentences

[pbfulmar]

Hi,
integrated a NKE instrument system with SignalK.
Added the following sentences (all sent by the NKE Multigraphic) to the NMEA parser:

• MTA, mean temperature of air
• MWD, wind speed and direction
• VWT, true wind angle and speed

Thanks for all your great work with SignalK!

[tkurki]

Thanks for the pr - looks pretty good, some minor review comments.

[pbfulmar]

Thank you for your kind reply and the detailed and helpful review!
I simply started with copy-paste of some existing modules since I couldn't tell which checks are eventually already done somewhere else, maybe by the parser.

The new commits include:

• checks for the units/designator fields in the MTA, MWD and VWT sentences
• converted values (from knots or km/h) are only used, when no value in m/s is present in any of the three sentences
• tests for the pathes are changed to positional references and a set of more systematic tests

This is my very first pr in github :-) should I click on the "resolve conversation" buttons or are you going to do that?
Viele Grüße
Peter

[tkurki]

And a fine one it is!

I have absolutely no idea if the fields can be in different order, but when not knowing exactly maybe the extra tests are in order.

If you want to tweak this to perfection in the ideal world this would be three separate commits, that each add one sentence, but I can merge it like it is and just squash all the commits to one. I don't want to see the whole story, just the end results in master's commit history.

[pbfulmar]

Thank you!
And I will remember to keep any future pr as small as possible :-)

Regarding the order of the fields: at least for MWD it must be fixed because 'M' is used for both 'magnetic' as well as for 'm/s'. In different order it would be ambiguous. Better don't trust any input data - so I think the extra tests are definitely in order.

Anything else you'd like me to do?