add 2 test headers

add tests (they fail)

getting closer to a solution

wipwipwip

today's WIP submission

more intermediate work.  I'll squash these commits, but I don't want to
delete them entirely as there is legitimate "by-hand" work hidden in
here

I think it works!

There is one conversion error that is ~a part in 10^7, which is probably
too big to be acceptable, but may reflect errors up the chain rather
than here
added ctype determinator
finally, something that's actually usable
added a note about the header
address @astrofrog's initial comments
added roundtripping test.  Fails!
fix a bug with the way the type dicts are specified

WIP.  I need to draw some diagrams here; there is always at least one
conversion that is not right in the round tripping tests, which implies
that there are many others uncaught.

WIP commit; this time a lot of success

WIP commit; this time a lot of success
all the logic is right, there's just one precision-based exception
refactor: make variable names symmetric, remove unnecessary function
update tests to slightly higher precision and to use the erratum's CDELT
corrections
a little more cleanup