Add XS_Kerrconv model to convolutional.jl

[GloriaRAH]

Adding a new convolutional model XS_Kerrconv

[fjebaker]

Test failing is not your fault

multifits: Error During Test at /home/runner/work/SpectralFitting.jl/SpectralFitting.jl/test/runtests.jl:96
  Got exception outside of a @test
  LoadError: MissingBackendError: Failed to use AutoReverseDiff().
  Backend package is probably not loaded. To fix this, try to run

      import ReverseDiff

  Stacktrace:

I will investigate this shortly.

[fjebaker]

Tested with

using SpectralFitting, Plots
model = XS_Kerrconv()
es = collect(range(0.1, 10.0, 200))
fs = allocate_model_output(model, es)
fs[100] = 1
plot(es[1:end-1], invokemodel!(fs, es, model))

Works fine!

[fjebaker]

I will merge this and then try and find out why the test suite is broken.