Only FFT each Pk once

[jablazek]

This may require a moderate re-factor. Currently, Pk gets an FFT each time it is called. We should instead create a Pk object (including P_window and C_window effects) that stores the coefficients and is then used by the main fast-pt functions.

Was there a reason we didn't do this already?

[jablazek]

@xfangcosmo : assigning this to you as well. For some reason, github doesn't let me directly assign it.