ENH: Allow required value in reportlet configs.

[rwblair]

Interprets 'required' key when present in a reportlets config dictionary.
If its true and no files/figures were found then raise an exception. Report indexer will catch these exceptions and raise a single exception at the end of its run with all of the collected reportlet exceptions.

fixes #78

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 28.90%. Comparing base (6404af3) to head (e9b3a82).
⚠️ Report is 299 commits behind head on main.

Files with missing lines	Patch %	Lines
nireports/assembler/report.py	85.71%	3 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (6404af3) and HEAD (e9b3a82). Click for more details.

HEAD has 113 uploads less than BASE

Flag	BASE (6404af3)	HEAD (e9b3a82)
	120	7

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #122       +/-   ##
===========================================
- Coverage   60.34%   28.90%   -31.45%     
===========================================
  Files          20       21        +1     
  Lines        2242     2266       +24     
  Branches      399      401        +2     
===========================================
- Hits         1353      655      -698     
- Misses        795     1595      +800     
+ Partials       94       16       -78     

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