Make name of field match snake case version of class name

[mwcraig]

For reasons I cannot recall, the name for the SourceLocationSettings in PhotometrySettings is source_locations. For all of the fields in PhotometrySettings the name of the field is the snake case version of the class name. So PhotometryApertures is photometry_apertures, etc.

It will turn out to be convenient for the last set of widgets if the name is always the snake case version o f the class name.]

This PR does that.

@JuanCab -- it would be great if this could get a quick review.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.67%. Comparing base (3fb5961) to head (318385a). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #370 +/- ## ========================================== + Coverage 77.56% 77.67% +0.10% ========================================== Files 27 27 Lines 3629 3628 -1 ========================================== + Hits 2815 2818 +3 + Misses 814 810 -4 ```

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