dev: fix/add some test context methods

* also move move all im3d core tests together
Teach im3d to accept additional attributes via ...

* and include a test
Fix docs for (as.)im3d
add basic implementation of materials functions
Merge branch 'feature/im3d-extra-attrs'

* feature/im3d-extra-attrs:
  add basic implementation of materials functions
  Fix docs for (as.)im3d
  Teach im3d to accept additional attributes via ...
  dev: fix/add some test context methods