Unify all __future__ imports
Won't validate against negative Mach number anymore
Added info about notation in gasdynamics module
Small improvements in docstrings and exception messages
Replaced incident with upstream, much clearer.
Added Mach angle and Prandtl-Meyer function in isentropic
And make shocks use them for convenience.
Removed useless docstrings from tests
Yet another change, hopefully definitive: finally A_Astar
Added also on the notation paragraph and made the test that ensures no
exception is raised if area ratio for M=0 is asked, see #1.
Trivial change of order in ObliqueShock