Option for multiple images per staged target

[e-koch]

Allow multiple line names/variations to be linked back to a single staged MS "parent" line name.

The use case here is for (a) staging with many MSs where duplicating the staging adds a lot of compute time and (b) producing multiple images with different gridding settings (e.g., different weighting, tapering, etc). Both are relevant for LGLBS and a number of new PHANGS/PHANGS-adjacent ALMA projects with multiple 12m configs.

[e-koch]

e.g., the parent line name is 12co21, with an imaging names of 12co21_robust_1.0, 12co21_1as_taper.

Each imaging name specifies a different imaging template file. The imaging names then need to be linked back to the parent line name to link back the right staged MS (similar to how we define array combinations)