Fix Gaussian Shape docs

[sjperkins]

Fixes #207

• Tests added / passed

  $ py.test -v -s africanus

  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 africanus
  $ flake8 africanus
  $ pycodestyle africanus
  

• 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]

Ignore my previous comment, I need to put the scaling factor in.

[sjperkins]

Documentation now looks like:

Do you have a reference for the emaj/emin/position angle conventions?

[landmanbester]

Sorry, I think I pointed at the wrong line. In the code, v1 is defined as

https://github.com/ska-sa/codex-africanus/blob/d4e4c01de1029e418f03c5fb096829a3202986b2/africanus/model/shape/gaussian_shape.py#L45

So there is a plus instead of the minus as indicated in the docs. Also, the effect of multiplying u1 by the ratio r is simply to replace emaj with emin so I am not sure what the point of r really is. Is this parametrisation correct? I would have to work out what the general parameterization of a 2D Gaussian is in Fourier space, don't know it off the top of my head. It seems to be correct for

.

Also, we should document the fact that emaj and emin refer to the FWHM of the Gaussian not the standard deviation to clarify the factor of