Move layer related code away from `interface.py`
Currently, interface.py
is a very large file. It would maybe be an idea to move some things away, and the layers plus all eps1 and mu1 classes come to mind. Potentially only the eps1 and mu1 classes as well, this would make it very easy for anyone to submit new ones (which I think should be encouraged) without conflicting with other code, since the eps1 and mu1 classes are very unlikely to change much.