Proposed minimal banding change to prevent fewer than 7 ionization bands

[jhmatthews]

This pull request (PR #1101) introduces updates to the banding strategy and other minor changes. Specifically:

• Enforcing a minimum of seven bands for ionization, otherwise defaulting to the old 10 hardwired bands we used to use
• New function ion_bands_init to replace freqs_init and ensure the ionization banding is carried out in a separate function
• Small adjustments to flags and comments to clarify the behavior of the bands in specific cases.
• New Function: check_appropriate_banding provides some rudimentary checks on banding, but could be expanded

Testing ongoing.

[jhmatthews]

Regression tests look good for this - I checked that nothing changes when you use TDE and AGN banding with matrix_pow. Things do change when you use TSTAR or CV banding with matrix_pow (as I expected from my changes), but overall the results look better, so I'm merging. The reporting is also helpful, if rudimentary.