fix bug in writing AASTex tables

The old implementation did not honor `latexdict['tabletype']` for AASTex tables
due to the order in which the various defaults were set.
This fix restores that and makes it possible to write e.g. 2-column
deluxetable* tables.
Small changes in existing tests are sufficient to make sure this bug will not
creep in again.
Improve test coverage in astropy.units.  Fix some bugs discovered along the way.
Fix test after rebase
Merge pull request #1372 from hamogu/fixlatex

fix bug in writing AASTex tables
Merge pull request #1405 from mdboom/unit/coverage

Improve astropy.units test coverage
First attempt at IERS A, B table classes, interpolation of UT1-UTC.
Cleanup of logical vs integer, use of super.

Correct bug introduced with previous commit
Memoize IERS_A and IERS_B: tables can be kept for session.
Initial linkage to time, with only simple exception messages.

Correction to linkage to Time
First test cases -- if for nothing else than stop me from some silly mistakes
Cleanup of interfaces

Cleanup of interfaces
Moved IERS-B data and both readme's to data subdirectory
Adapt setup_package.py to include iers/data files
Add documentation, some tests, ensure ReadMe's work on .gz files
Simplified Time part; added IERS A tests; document how to use A if needed
Remove documented trickery that's unlikely to remain (or be needed).
Next iteration: exception in ut1_utc, safer memoization
Corrected doctest examples
Provide get_delta_ut1_utc method to Time; update documentation
Updates of documentation and doctests / simplification of get_delta_ut1_utc