add validation function for psf norm

[AMWMitchell]

No description provided.

[codecov]

Codecov Report

Merging #142 (b61c2fb) into master (aa63141) will decrease coverage by 0.78%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   87.35%   86.57%   -0.79%     
==========================================
  Files          38       39       +1     
  Lines        1329     1341      +12     
==========================================
  Hits         1161     1161              
- Misses        168      180      +12     

Impacted Files	Coverage Δ
pyirf/validation.py	0.00% <0.00%> (ø)

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[HealthyPear]

Thanks for this!

I think the code is OK, but before approving I would like this PR to have 2 small additional things,

• a unit-test under pyirf/tests/test_validation.py in which a PSF table not completely normalized is fed to the validate_psf which asserts the failure
• a section in the docs at the same level of e.g. docs/utils.rst called docs/irf_validation.rst or docs/output_validation.rst in which this contribution is explained - then we can use that docs page to describe any other DL3 validation tool we might introduce (you can copy/paste/edit any docs section at the same level to get the same layout)