This PR updates the lqe() function to allow it to be called with a StateSpace system as the first argument rather than the matrices that define the system. This matches the functionality of the lqr command. Docstrings have been updated to reflect the new functionality and to match the notation used for lqr a bit better. Unit tests added to cover the new code.

This PR has some overlap with #670, so depending on which gets merged first it may be necessary to rebase.