Processing 3D cubes with FFCLEAN

I've just modified kappa:ffclean so that it can:
1) process 3D cubes. It will do this either by processing the cubes as a set of independent 1D spectra, or as a set of independent 2D images (see new parameter AXES)
2) store the calculated noise level in the output variance array (see new parameter GENVAR)

This was motivated by my experiments with the new smurf:unmakecube command as a means of getting an estimate of the noise level in each residual spectrum.