Remove xarray from testing dependencies

[sjperkins]

• Removes full xarray testing dependency introduced in Add experimental code for data fragments. #282.

• Fixes some test cases to work without xarray.

• 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