Generalise script to accept arbitrary number of collects

[ShashankBice]

Right now, the triplet stereo modes expect 3 collects. It would be good to generalise it to accept n number of collects. This would help if we have a bi-stereo vs a tri-stereo, or have 2 or more triplets collected over the same spot on the ground within a reasonable timespan.

Scripts which will not change:

• skysat_overlap.py
• skysat_preprocess.py
• ba_skysat.py
• orthorectification step in skysat_orthorectify.py
• Different modes in skysat_pc_cam.py

Scripts which will change:

• skysat_stereo_cli.py
• skysat_dem_mos.py
• mosaicing steps in skysat_orthorectify.py