ENH: Allow different types of reportlets, not only BIDS-based

[oesteban]

Adds two new types of reportlets:

• metadata, which basically renders an input dictionary (possibly nested) formatted as a table (instead of just receiving a ".html" reportlet).
• custom, which currently hard-wires two subtypes of custom reportlets: the error summary and the boilerplate.

Limitations:

• the interface of the metadata reportlets is still unclear, will continue defining it in the future with the migration of MRIQC
• the configurations of custom reportlets are a bit clunky, but I believe they still add flexibility over what we had.

Extra:

• Since the output directory can be set from outside completely, reports should now support YODA (see https://github.com/nipreps/fmriprep/blob/master/fmriprep/reports/core.py#L27-L29 /cc @effigies)
• Added some tests

Examples:

• Report with errors and boilerplate specifications that could not be rendered
• Report with errors and boilerplate specifications, one crashfile placed at the right location and is rendered
• Report with errors and boilerplate specifications, boilerplate placed at the right location and is rendered

Towards: #57.

[codecov-commenter]

Codecov Report

Patch coverage: 92.74% and project coverage change: +3.46 🎉

Comparison is base (95dff6d) 51.50% compared to head (dff5659) 54.97%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
+ Coverage   51.50%   54.97%   +3.46%     
==========================================
  Files          17       17              
  Lines        1689     1788      +99     
  Branches      337      359      +22     
==========================================
+ Hits          870      983     +113     
+ Misses        743      728      -15     
- Partials       76       77       +1     

Flag	Coverage Δ
unittests	54.62% <92.55%> (+3.35%)	⬆️

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

Impacted Files	Coverage Δ
nireports/assembler/tools.py	31.25% <ø> (ø)
nireports/assembler/report.py	90.90% <86.66%> (+10.72%)	⬆️
nireports/assembler/reportlet.py	92.85% <94.11%> (+3.96%)	⬆️
nireports/assembler/misc.py	79.76% <95.65%> (+24.65%)	⬆️

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[effigies]

The overall changes to the reporting code are very difficult to assess. Reviewing test_report.py, the changes to the API seem like manageable generalizations.

I have reviewed the three sample reports in the CircleCI output, and they appear reasonable.

One small comment about a function that I couldn't quickly work out the behavior of by reading the code.

[oesteban]

Beefed up documentation and coverage, not limited to the unfold_columns() function. Merging -- thanks for the feedback!