WIP: add conversion between *LINEAR* spectral coordinate systems.  Leaves
MAJOR todo items open.  Not fully implemented due to bug in wcslib or
astropy.wcs
plane/airport work on spectral axis conversion.  Definitely a continued
WIP.
some renaming and refactor toward usefulness
another intermediate commit
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
***THIS COMMIT SHOULD BE REVISITED AT SOME POINT***
This is a hack to keep the tests passing; it changes the CRVAL3Z values
to be what I compute them to be using 64 bit precision (give or take),
rather than what Greisen reports.  We still ought to track down this
error, even if it means going to Greisen and Calabretta directly, which
may be a logical next step
py3: items not iteritems
py3: no message.message
Merge pull request #58 from keflavich/spectral_axis

Convert spectral coordinate systems (velo <-> freq)