Integrate qsymm into kwant

Adds functionality by @dvarjas and me that allow qsymm to interface with Kwant systems, and vice versa. These functions make it possible to convert Kwant builders to qsymm Model objects and back, facilitating symmetry analysis of Kwant systems.

This MR depends on qsymm features that are not included in v1.0. Before merging, we thus need to release an updated version of qsymm.

closes #256 (closed)