Improve MS structure inspection

[sjperkins]

• Tests added / passed

  $ py.test -v -s daskms/tests

  If the pep8 tests fail, the quickest way to correct
  this is to run autopep8 and then flake8 and
  pycodestyle to fix the remaining issues.

  $ pip install -U autopep8 flake8 pycodestyle
  $ autopep8 -r -i daskms
  $ flake8 daskms
  $ pycodestyle daskms
  

• Fully documented, including HISTORY.rst for all changes
  and one of the docs/*-api.rst files for new API

  To build the docs locally:

  pip install -r requirements.readthedocs.txt
  cd docs
  READTHEDOCS=True make html
  

[sjperkins]

Currently, Column Shape is inferred by reading exemplar rows for each dataset. This could potentially be expensive in the case of 32k channels and otherwise wastes disk I/O.

Column shape is encoded in various MS sub-tables. This PR contains some exploratory coding for reading the various NUM_* columns in the sub-tables and, in particular determining the number of channels and correlations for each DDID.