Fix #199.

[JSKenyon]

• 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
  

This changes the behaviour of xds_to_zarr. A new dataset will be constructed after selection of data_vars and coords but before construction of the schemas. This means that selecting columns (data_vars) will no longer produce partially initialised values for unselected data_vars/coords. This also begins addressing #171 and #118 (for zarr) - more work will still be needed to reconcile chunking and propagate the changes to the casa/parquet interfaces.