Improve sub-table creation logic

[sjperkins]

• [x] Tests added / passed

  $ py.test -v -s xarrayms/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 xarrayms
  $ flake8 xarrayms
  $ pycodestyle xarrayms

• [x] 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]

In python-casacore, opening WSRT.MS::SOURCE opens the SOURCE via the WSRT.MS table, while WSRT.MS/SOURCE opens the SOURCE table directly.

The first approach can result in the sub-table's locks interacting with those of the main table, preventing writes in particular.

Internally, dask-ms uses the second approach since we're not supporting TAQL queries. However, dask-ms still expects the first approach in the xds_{from,to}_table interface as it informs dask-ms about the link between the main and sub-table. In particular, this is used to transparently place a "Table: " keyword in WSRT.MS to inform it that "SOURCE" is now a subtable.

Table path inspection has also been cleaned up and made more consistent.