PICARD: new recipe to calculate and analyze spatial power spectrum

Calculate and analyze the azimuthally-averaged spatial power spectrum
for SCUBA-2 images. The image is trimmed to remove noisy edges and the
source emission masked out before calculating the power spectrum.

This spectrum is analyzed to determine the angular scales
corresponding to the peak and half power scales, and reports the mean
power on various length scales up to the filtering scale used when the
map was made (filt_edge_largescale).