Overhaul l2interpolationerror.hh: more comments, better variable names, fixed includes.
Reflect the new locations and names of constraints headers.

Thus, produces only a zero solution.
Fix another deprecated constraints include.
Merge branch 'feature/move-constraints-headers'

Adapt to the move of the constraints headers in PDELab.

Kudos to Christoph for doing all the work!

* feature/move-constraints-headers:
  Fix another deprecated constraints include.
  Reflect the new locations and names of constraints headers.