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

Attention: Patch coverage is 90.32258% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 59.00%. Comparing base (60fd854) to head (626b07b).

❗ Current head 626b07b differs from pull request most recent head 131d1a5

Please upload reports for the commit 131d1a5 to get more accurate results.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   58.34%   59.00%   +0.66%     
==========================================
  Files          22       23       +1     
  Lines        2096     2120      +24     
  Branches      365      367       +2     
==========================================
+ Hits         1223     1251      +28     
+ Misses        788      786       -2     
+ Partials       85       83       -2     

Flag	Coverage Δ
unittests	58.80% <90.32%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.