Merge branch 'modes' into devel

TODO

doc/other/linear_system.pdf

doc/other/reduce_modes.pdf

doc/source/reference/kwant.builder.rst

doc/source/reference/kwant.physics.rst

doc/source/whatsnew/0.3.rst

examples/square.py

examples/tests/test_square.py

kwant/builder.py

kwant/physics/__init__.py

kwant/physics/leads.py

kwant/physics/tests/test_leads.py

kwant/physics/tests/test_noise.py

kwant/solvers/common.py

kwant/solvers/tests/_test_sparse.py

kwant/solvers/tests/test_mumps.py

kwant/solvers/tests/test_sparse.py

kwant/system.py

kwant/tests/test_system.py