model fitting: ensure specutils has access to equivs & set targ solid angle

[gibsongreen]

This draft PR resolves:

1. UnitConversionError in Model Fitting plugin, specutils spec.with_flux_unit did not have access to pix2 custom unit equivalencies

To reproduce on main:

uri ='https://data.science.stsci.edu/redirect/JWST/jwst-data_analysis_tools/IFU_cube_continuum_fit/NGC4151_Hband.fits'

model = cubeviz.plugins['Model Fitting']
model.cube_fit = True
model.create_model_component()
model.calculate_fit()

2. TypeError: unsupported operand type(s) for *: 'CompositeUnit' and 'NoneType'. When toggling 'Cube Fit' in Model Fitting plugin, solid_angle_in_targ was not being set after targ_units was update to include a solid angle unit. Setting 'targ_units' triggered subsequent indirect conversion logic. By setting the solid_angle_in_targ we follow the same logic route and ensure we maintain output units in surface brightness.

To reproduce:

# example notebook cube
uri = "mast:JWST/product/jw02732-o004_t004_miri_ch1-shortmediumlong_s3d.fits"

uc = cubeviz.plugins['Unit Conversion']
uc.flux_unit = 'erg / (Angstrom s cm2)'

model = cubeviz.plugins['Model Fitting']
model.cube_fit = True
model.create_model_component()
model.calculate_fit()

Description

Change log entry

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  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (6c946d3) to head (ed72509).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3343   +/-   ##
=======================================
  Coverage   88.80%   88.80%           
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  Files         125      125           
  Lines       19137    19158   +21     
=======================================
+ Hits        16995    17014   +19     
- Misses       2142     2144    +2     

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