add simplify identities #529
                
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Hi, well this time i propose implement in simplify the expand_func, its very useful specially if we use special functions like beta or gamma, in this case the user can choose how treat the function, in sympy we have 3 options to simplify, "simplify", "combsimp", and "expand_func", actually simplify can cover the combsimp functions, so we only add two.
maybe use directly:
instead of choose the method, what do you think?
the tests will fail until merge this: #526
Cya.