Fix CONSTRAINTS_DOC formatting in modeling docs
Add default config values to FakeApp allowing tests to run without crashing on missing config attributes.
Update test to reflect new expected reults since the changes in 5da52803d2d9dd3a5c2663353899bd04655ab53a
A few miscellanous tweaks to the test runs:
 * Fix an issue where an error could occur when running setup.py test
   with the -P option but there are no Sphinx docs for the selected
   package
 * It turns out running pytest_ignore_collect causes paths to be
   ignore for all of pytest, not just a single plugin.  Kind of a pain,
   that.  This tweaks how DoctestPlus uses pytest_ignore_collect to
   implement the doctest_norecursedirs option.
Update operations.rst

Fixed typo.
Fixed double backticks
Fixed group_by references
Fixed Table and Column references
Moved ParametricModel._model_to_fit_params to being a static method in Fitter;
this is the first step in separating concerns so that only Fitters are
responsible for getting their inputs into the correct format they need.

This allowed elimitating a lot of code rot in the process.

This did require outright removing a handful of test cases, but those tests
would be better handled within the fitter tests anyways--simply marking some
parameters as constrained should have no bearing on a *model's* behavior by
itself.
Fixed Gaussian2D model for cov_matrix input
Added change log entry for #2199
Added regression test for #2199
Changed Gaussian2D model rotation to increase counterclockwise
Updated regression tests for #2199
Updated documentation for Gausian2D model
Updated change log entry for #2199
Updated docs to display the covariance matrix as a matrix
Explicitly raise NotImplementedError for _repr_latex on Quantity arrays
Added HTML reader to io.ascii
Added reader to existing system