make is_differentiable as attribute

[Borda]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Implements #544 (comment)
cc: @SkafteNicki

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #551 (dedb16b) into master (fbdc3eb) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #551   +/-   ##
=====================================
- Coverage      95%    95%   -0%     
=====================================
  Files         130    130           
  Lines        4690   4618   -72     
=====================================
- Hits         4472   4397   -75     
- Misses        218    221    +3     

[tchaton]

LGTM !