Modernize the electrodynamic (Nedelec) operators

This is a massive refactoring to make the code a bit more readable and to employ modern C++ features, such as range-based for loops. It brings increased composability by accepting callables as parameter functions. The old interface with parameter classes is deprecated.

Also included is a unit test adapted from code by Andreas Buhr.

Fixes: #24 (closed)

Note: #24 (closed) mentions "check for new entity interface", I'm not sure what is meant there. I'm assuming it isn't really an issue. Reopen #24 (closed) or open a new bug report if you have specifics.