Add support for generating observations for non depth variables

[selimnairb]

Implements initial support for non-depth variables, currently, water temperature. Support for other NMEA0138 and NMEA2000 variable can be added in the future. This PR also adds a vbi command line tool that features a dump command to write ASCII versions of logger data files supported by OpenVBI to the screen or to a file. This is useful for debugging (I used it to help develop non-depth variable support).

[brian-r-calder]

I think I would prefer the "todo" work in observations.py be in a separate file, but otherwise this should be good.

[selimnairb]

I think I would prefer the "todo" work in observations.py be in a separate file, but otherwise this should be good.

How about I create a directory called docs/dev and therein place a file called TODO.md that has the notes on the NMEA2000 and NMEA0183 messages we'd like to add support for in observations.py?

[brian-r-calder]

I think I would prefer the "todo" work in observations.py be in a separate file, but otherwise this should be good.

How about I create a directory called docs/dev and therein place a file called TODO.md that has the notes on the NMEA2000 and NMEA0183 messages we'd like to add support for in observations.py?

That'll work, I think.