Remove 'constant ionization source'

[johnomotani]

This functionality has been replaced with the more flexible 'external sources' feature, so can now be removed.

Also removes the 'Harrison-Thompson' debug check as this now includes no features that are not in the 'recycling fraction' check.

[mrhardman]

Could you please document the options required to reproduce the constant ionization source (if you haven't already done so), and then post a link here to where I can find it?

Thank you! (This will remove another set of options : ) ).

[johnomotani]

The external source options are documented in the documentation
https://mabarnes.github.io/moment_kinetics/dev/external_sources_notes/

The harrisonthompson.jl test has settings to reproduce the old 'constant ionization source' with 'external sources':

ionization_frequency = 0.688

test_input_finite_difference = Dict(
                                    ...
                                    "ion_source" => Dict("active" => true,
                                                         "source_strength" => ionization_frequency,
                                                         "source_T" => 0.25,
                                                         "z_profile" => "constant",
                                                         "r_profile" => "constant"),
                                   )