make sure we only perform absolute imports on loading a backend

Fixes #2309
Add a comment explaning __import__ syntax.
[bug correction] triinterpolate is independant of triangulation numbering.
Merge pull request #2363 from GBillotey/new_test_triinterpolate

[bug correction] trirefine is now independant of triangulation numbering
Merge pull request #2335 from andreabedini/patch-5

make sure we only perform absolute imports on loading a backend