Array combination (12m + 7m + TP) for HCO+

[d-l-walker]

This issue is for discussing progress and updates on combined images for HCO+.

To get a first look, I've done a simple feather of all three arrays, as we've been doing for HNCO (see #384 for discussion). Feathered 12m + 7m + TP cubes can be found on globus in /upload/HCOp_feather_images/.

Currently fields al (#126) and am (#259) are not included, as they require a couple of extra steps due to being imaged manually rather than by the PL.

@keflavich I've also added the corresponding 12m weight cubes in /upload/HCOp_feather_images/HCOp_12m_weights/. Would it be possible to get the mosaicking running for this? Alternatively, we could wait until the two remaining fields are done so that we have all regions included.

[d-l-walker]

I haven't looked at the combined images in much detail yet. One thing that I noticed though is that field aq (which includes Sgr A*) has a small region of masked pixels in the combined cube (in all channels, see peak intensity image below). After some digging, I realised that this is coming from the Total Power data -- the cube that was delivered to us contains these masked pixels (see #223 (comment)).