Follow formulation in terms of the cumulant expansion

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

Refactor integration

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.

Docstring update

Minor docstring fix.

Non-traceless bases and housekeeping

The main change is the possibility to use bases with more than one traceless element. This allows for separating subspaces, see PR #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.

First release

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