rename self_energy to selfenergy

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/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