#64 Read FITS cubes for prediction

[André Offringa]

In O. Smirnov's words:

It would be very nice if -predict only mode could handle 4D FITS
cubes (i.e. with FREQ and STOKES axes). Now I realize there's good
reasons why the _output_ of wsclean is a series of 2D FITS files
rather than a cube (irregular frequency axes and all that), but would
it be difficult to implement a 4D special case just for -predict
input?