integrate discretizer into kwant

.gitlab-ci.yml

discretizer/__init__.py

discretizer/algorithms.py

discretizer/discretizer.py

discretizer/interpolation.py

discretizer/postprocessing.py

discretizer/tests/test_algorithms.py

discretizer/tests/test_hoppings.py

kwant/__init__.py

kwant/continuum/__init__.py

kwant/continuum/_common.py

kwant/continuum/discretizer.py

kwant/continuum/tests/__init__.py

kwant/continuum/tests/test_common.py

kwant/continuum/tests/test_discretizer.py