Fixes from_corner/extrude for mixed single/multi-band materials

[fakufaku]

Fixes issues #381.

When creating a room with from_corners with multi-band material, and then making it 3D with extrude with a single band material for the floor and ceiling, an error would occur.
After the fix, the materials for floor and ceiling are automatically extended
to multi-band, as expected.

• Are there docstrings ? Do they follow the numpydoc style ?
• Have you run the tests by doing nosetests or py.test at the root of the repo ?
• Have you checked that the doc builds properly and that any new file has been added to the repo ? How to do that is covered in the documentation.
• Is there a unit test for the proposed code modification ? If the PR addresses an issue, the test should make sure the issue is fixed.
• Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased".