#93a Fix TreeCorr imports and 1d arrays in FITS table writing

[msimet]

Hi folks,

Two cleanup items in this PR:

• We say in the README that TreeCorr is a good idea, but not required. That wasn't actually true. This fixes the rogue import unprotected by a try block.
• The FITS standard allows for columns where each row of the column is actually a 1D array. The way I coded up to turn numpy arrays into FITS tables didn't allow for that, though. This fix does allow it.