It used to be that ```MaterialLibrary``` only tried to handle .hdf5 and .h5 files. In this PR I added .h5m files to this list. However, once I did that this line (line 594) started breaking tests because not all .h5m files have materials (```MaterialLibrary``` could not find ```/materials``` within the .h5m). Basically what we need here is a check to see if ```\materials``` exists before assigning ```self.mats```. Note that further down in the file ```self.mats``` gets reassigned, so my changes did not break any original tests. All of this logic should probably be consolidated.