move Site and SiteFamily to 'system.py'

doc/source/pre/whatsnew/0.2.rst

doc/source/pre/whatsnew/1.0.rst

doc/source/reference/kwant.builder.rst

doc/source/reference/kwant.system.rst

doc/source/tutorial/first_steps.rst

doc/source/tutorial/operators.rst

doc/source/tutorial/plotting.rst

doc/source/tutorial/spin_potential_shape.rst

kwant/builder.py

kwant/continuum/discretizer.py

kwant/continuum/landau_levels.py

kwant/kpm.py

kwant/lattice.py

kwant/operator.pyx

kwant/physics/gauge.py

kwant/plotter.py

kwant/system.py

kwant/tests/test_builder.py