Filter-functions

The main change is the possibility to use bases with more than one traceless element. This allows for separating subspaces, see PR https://github.com/qutech/filter_functions/pull/11.

Additionally, some housekeeping and performance improvements have been made. Most of the performance gain was achieved by calculating the real and imaginary parts of a complex exponential separately.

Note that this package is still in alpha stage and thus subject to changes that may break backwards compatibility.