Parse NMEA tag blocks

[squidpickles]

An example NMEA message with tag block might be

\s:2573435,c:1699169531*03\!BSVDM,1,1,,B,13md`u0P00PoLB4V`C8=;wvF24r0,0*0F

It would be nice to parse these, rather than just ignoring them (or worse, erroring)

[schwehr]

My libais code has lots of tag block examples:

• https://github.com/schwehr/libais/blob/d738df7c508af467410c7537229673aa3c3b44c8/test/nmea_queue_test.py
• https://github.com/schwehr/libais/blob/d738df7c508af467410c7537229673aa3c3b44c8/test/tag_block_test.py
• https://github.com/schwehr/libais/blob/d738df7c508af467410c7537229673aa3c3b44c8/test/nmea_test.py

Especially important are the group \g tags.