How to move general routines from stix branch to SSW gen folder

[samaloney]

@annavolp and I have fixed a small bug in 'vis_fwdfit_pso', now it works fine.
Just a couple of comment:

• 'vis_fwdfit_pso' calls 'cmreplicate' which is not implemented by us (we found it at 'https://lesia.obspm.fr/kronos/routines/cmreplicate-code.html'). We uploaded this routine on the repo since we did not have it. We cannot understand whether it is already in an existing ssw package;
• 'vis_fwdfit_pso' is a general routine, maybe it can be moved (together with the sub routines) in 'ssw/gen/idl/image/vis'. Is there a specific requirement that a routine has to satisfy for being moved to the 'gen' folder;
• 'swarmintelligence' is a routine for performing Particle Swarm Optimization, hence is not specific for STIX or, more in general, for image reconstruction. Maybe it could be moved in a more suited ssw folder.

Originally posted by @paolomassa in #52 (comment)

[samaloney]

Also relevant for #223 as this adds external files

[ennosigaeus]

Same goes for this file stix/idl/processing/spectrogram/mk_rate_hdr.pro, also see PR #223