ENH: Speed up CUDA resampling, reduce GPU memory reqs

This speeds up CUDA resampling by 10-20%, and reduces memory requirements for performing the resampling. Also, added a test. 

I noticed that the resampling is only good to 1 decimal place (for a given long `randn()` vector) due to slight differences between CUDA and FFTPack's FFT routines. Looking at the signals, they are very nearly identical. Looking at the FFT of each resulting signal, there appear to be very slight differences in the DC and Nyquist freq values only. I think it should be okay...