Support for FFTs

This wraps the FFTW library to do FFTs in libdynd.
I did nd::dtyped_empty as a version of nd::empty which just accepts the ndim/pointer pair, you should be able to just use nd::empty for this (and make the vector<> version an nd::empty overload). Do you know of a way to avoid the ambiguity of NULL between 0 and a pointer to avoid this workaround?