MAINT: fixed a type issue that occured with some python versions when calling `build/find_optimal_cell_shape`

The line search_range -= 1.0 in find_optimal_cell_shape was only inserted to satisfy the pyflakes/pep8 test. search_range, however, is in principle an integer numpy array, which caused exceptions on some machines/python versions. I therefore merely changed the line in question to search_range = -1.

At the same time I fixed a minor error in the documentation as the arguments of the make_supercell function at the end of defects.rst were in the wrong order.