minor formatting, indentation, and style fixes

kwant/graph/c_scotch.pxd

kwant/graph/c_slicer.pxd

kwant/graph/core.pyx

kwant/graph/tests/test_dissection.py

kwant/graph/tests/test_scotch.py

kwant/graph/utils.pyx

kwant/lattice.py

kwant/linalg/_mumps.pyx

kwant/linalg/cmumps.pxd

kwant/linalg/f_lapack.pxd

kwant/linalg/lll.py

kwant/linalg/tests/test_linalg.py

kwant/physics/leads.py

kwant/plotter.py

kwant/solvers/common.py

kwant/solvers/sparse.py

kwant/solvers/tests/test_sparse.py

kwant/system.py

kwant/tests/test_builder.py