Fixing the interpolation scheme for 3D power

[qezlou]

• If the spectra is not on a unifrm grid, calculates the average delta_F on a unifrom grid with the same resolution as the transverse direction
• The power for a grid with the resoltion of (250, 250, 105) ckpc/h will be 5-10% different from a unifrom grid of (105, 105, 105) ckp/h.
• So it would be ideal if someone uses a higher resolution unifrom map from the begining