Remove reports module in favor of pybids-reports namespace package

[tsalo]

Closes #801. This PR removes everything related to the reports module, since we will move to using a namespace package called pybids-reports. This will remain a draft until I've addressed the to-do list below.

To do:

• Document the namespace package
• Release pybids-reports (tracking with Release and push to PyPi pybids-reports#17)
• Add some kind of check for the namespace package in pybids

[codecov]

Codecov Report

Base: 86.31% // Head: 85.59% // Decreases project coverage by -0.72% ⚠️

Coverage data is based on head (6c0c484) compared to base (af095cc).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
- Coverage   86.31%   85.59%   -0.72%     
==========================================
  Files          35       30       -5     
  Lines        4033     3665     -368     
  Branches      976      903      -73     
==========================================
- Hits         3481     3137     -344     
+ Misses        356      348       -8     
+ Partials      196      180      -16     

Impacted Files	Coverage Δ
bids/__init__.py	100.00% <ø> (ø)
bids/utils.py	67.36% <0.00%> (-18.95%)	⬇️
bids/layout/models.py	91.08% <0.00%> (-0.96%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[Remi-Gau]

OK so the pybids-reports has been released so we can now think of how to take it out of pybids proper.

@effigies do you want to go though a deprecation cycle for a couple of releases before we remove it with a warning to let people know about the new package?