Efficiency: calculate G only on mask

[bmorris3]

Suggestion from @jkentwallace: Currently, the Fourier transform of the impulse response function G, calculated in Hologram.fourier_trans_of_impulse_resp_func operates on the whole FT(hologram) matrix, but we only need to compute it on the non-zero portions of the Fourier mask.

Modify this line to compute the mask only on the portions that are non-zero in the Fourier mask.