Kahane's algorithm to remove contracted gamma matrices from expressions in four dimensions.
Algorithm is now explained in docstring, docstring now linked to sphinx. Files moved to the physics module. Minor fixes in source code.
test cases now have dimensional parameter, so they can later be re-used for dimensional regularization tests.
removed all try excepts from kahane_simplify function
fixing bug in previous commit
Merge pull request #2416 from Upabjojr/kahane

Kahane's algorithm for reducing contracted products of gamma matrices