Filter-functions

Hotfixes a bug when calculating the infidelity before the infidelity derivative (58).

This is the first stable release.

Major changes over v.0.4 include:
* Added calculation of second order Magnus expansion terms (46)
* Improved performance of gradient calculation (53)
* Updated documentation (44)
* Easier detection of a `notebook` environment (uses `ipynbname` as a new dependency) (50)
* Several small improvements.

This version adds the calculation of gradients courtesy of JDTeske and Isabel Le for use with `qopt` (living for now at [git-ce.rwth-aachen.de](https://git-ce.rwth-aachen.de/qutech/qopt)).

The package has been restructured to follow the theory's formulation in terms of the cumulant expansion.

Refactor the frequency domain integration.
* Toss duplicate code
* Introduce switch to integrate the error vector correlation functions <u_kl> in a loop over one index to save memory at the cost of computation speed.

Minor docstring fix.