update copyright notices

doc/Makefile

doc/source/conf.py

doc/sphinxext/kwantdoc.py

kwant/__init__.py

kwant/_common.py

kwant/_system.pyx

kwant/builder.py

kwant/digest.py

kwant/graph/__init__.py

kwant/graph/c_scotch.pxd

kwant/graph/c_slicer.pxd

kwant/graph/c_slicer/bucket_list.h

kwant/graph/c_slicer/graphwrap.h

kwant/graph/c_slicer/partitioner.cc

kwant/graph/c_slicer/partitioner.h

kwant/graph/c_slicer/slicer.cc

kwant/graph/c_slicer/slicer.h

kwant/graph/core.pxd

kwant/graph/core.pyx

kwant/graph/defs.h