Inconsistent documentation for Gaussian shape function

[landmanbester]

It has just come to my attention that the formulae in the docs (https://codex-africanus.readthedocs.io/en/latest/model-api.html) do not match up with what is actually computed in the function

https://github.com/ska-sa/codex-africanus/blob/d4e4c01de1029e418f03c5fb096829a3202986b2/africanus/model/shape/gaussian_shape.py#L49

Look at the expression for v_1 compared to how it is defined in the code. Also, the docs refer to lambda as frequency and I am pretty sure it should be wavelength. There is also no mention of the scaling function that compensates for the fact that e_{min} and e_{maj} actually refer to the FWHM of the components (which is not the usual way to parametrise a Gaussian). It might be wise to update the docs to explain this parameterization a bit better.