Foucalc: Export CPD calculation into new function

* Clean-up of foucalc code: export the cross-power-density computation
  into new function cross_power_density
* Also update help of dorDFT: It previously said that the
  normalization is 1/sqrt(2*PI) but I think this is incorrect. The
  normalization of the ISIS FFT is 1/sqrt(N) and dorDFT multiplies the
  output by N, thus the normalization is sqrt(N)