move kwant.InfiniteSystem.energies functionality to kwant.physics.Bands

doc/source/images/3-band_structure.py.diff

doc/source/images/4-graphene.py.diff

doc/source/images/5-superconductor_band_structure.py.diff

doc/source/reference/index.rst

doc/source/reference/kwant.physics.rst

doc/source/tutorial/3-band_structure.py

doc/source/tutorial/4-graphene.py

doc/source/tutorial/5-superconductor_band_structure.py

doc/source/tutorial/tutorial3.rst

doc/source/whatsnew/0.2.rst

kwant/physics/__init__.py

kwant/physics/dispersion.py

kwant/physics/tests/test_dispersion.py

kwant/system.py

kwant/tests/test_system.py