Merge branch 'stable'

.gitattributes

.gitlab-ci.yml

RELEASE.rst

doc/source/code/figure/kernel_polynomial_method.py.diff

doc/source/conf.py

doc/source/reference/kwant.rst

doc/source/tutorial/kpm.rst

doc/sphinxext/kwantdoc.py

kwant/_kwant_version.py

kwant/continuum/__init__.py

kwant/continuum/_common.py

kwant/continuum/discretizer.py

kwant/continuum/tests/test_common.py

kwant/continuum/tests/test_discretizer.py

kwant/lattice.py

kwant/physics/tests/test_symmetry.py

kwant/plotter.py

kwant/solvers/mumps.py

kwant/solvers/sparse.py

kwant/tests/test_builder.py