Merge pull request #2093 from larrybradley/airy

fixed AiryDisk2D model
Merge pull request #2248 from larrybradley/patch-3

Update equivalencies.rst
Spelling correction for UNIT_NOT_INITIALISED, PEP8 changes
(found by emacs showing nice pink lines for mistakes)
Add test cases for uninitialized unit
Rebuild from fresh master since rebasing was failing horribly.
Extra - got into title for 0.4
Merge pull request #2257 from mhvk/quantity-remove-the-pink

Quantity spelling correction for UNIT_NOT_INITIALISED, PEP8 changes
Fixed double backticks
Fixed group_by references
Fixed Table and Column references
Update index.rst

Fixed typo.
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
Merge pull request #2263 from larrybradley/patch-5

Update index.rst
Updated change log entry for #2199