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)

changed the description

Looks good overall. I will start with some global remarks below.

Is it correct that there is no functionality for specifying a discrete symmetry as calculated by qsymm to a builder?

If this is indeed not there, we should probably add it.

something like

qsymm_symmetries = analyse(syst)
update_symmetries(syst, qsymm_symmetries)  # Only provides the symmetries known by Kwant

We need to decide on the naming and namespace.

Several constraints come to mind:

• qsymm depends on sympy, so it must be lazily loaded, or not imported by default
• there is already kwant.physics.symmetry that has some related functionality

Packaging and testing infrastructure are missing right now. @jbweston please take a look whenever you have time.

changed the description

mentioned in merge request !260 (merged)

added 3 commits

• 2012cabe - add licensing information to new modules
• 76a1f381 - add __init__.py to qsymm subpackage
• 0e0ed683 - add qsymm as an optional dependency

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added 1 commit

• e146d8d0 - add qsymm as an optional dependency

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added 1 commit

• b17a6f0f - [SQUASH] fix trailing whitespace

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Also shouldn't we add a documentation page for this subpackage?

A final issue is style. Running pylint, pep8 and flakes8 on the source reveals things such as unused imports etc.

Maybe the best course of action is to fix up these things and squash them down into the original commit, given that there is only 1 functional commit in this MR (before all my packaging stuff)