Enables ccode(..., user_functions={..., 'Pow': ...})

pow(double, double) in math.h is not efficient for expressions like:
pow(1.14, 2) etc. where the exponent is a small integer. For other functions
one can use ``user_functions`` kwarg in ccode to give own versions. This PR
enables "Pow" to be a valid key too. (Note that fortran has language level support for Pow,
and hence need no such options).