update opd thermal slew test, in particular for a change in pupil segment data file

[mperrin]

As discussed & debugged on slack with @kjbrooks : a change in the jwpupil_segment.fits data file to more accurately handle pixels at segment boundaries (see #393) led to a failing test for the thermal slew OPD. It's not necessary to maintain precise agreement at floating-point machine precision for this, so the correct fix is to update the test code to a new expected value. I did this, with a check for the data file version to use the right value with or without the updated data files.

I also added a test check of the OPD's rms as well as the max, so that the test checks more than just a single pixel in the image. This too has an expected value that depends on which data file version is used.

For the record here's the slight difference in the two data files:

[codecov]

Codecov Report

Merging #396 (9ef7497) into develop (aaca6cf) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #396   +/-   ##
========================================
  Coverage    48.29%   48.30%           
========================================
  Files           14       14           
  Lines         5721     5722    +1     
========================================
+ Hits          2763     2764    +1     
  Misses        2958     2958           

Impacted Files	Coverage Δ
webbpsf/opds.py	38.50% <100.00%> (+0.05%)	⬆️

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