RAP-450: Add concatenation step

This MR adds a concatenation step before the imaging step to speed up imaging for data fractions < 1, avoiding the large penalty incurred when using multiple MS files (due to the facet that multiple measurement sets are individually gridded, and therefore every measurement set causes the FTs to happen). The existing concat_ms.py script was adapted for use in this step. A few issues with the script were fixed as well (e.g., the use of nargs="+" with two positional arguments).