Draft: WS-2759: Viewing a VLASS cube in CARTA (more fixes)

When trying to run CARTA with single_image being set to True for VLASS cubes, the only aspect that wasn't working was the name of the file to be opened by default in the CARTA session does not exist. This is because it is the name of the cube, which is not an actual file. Thus, if a valid file name is retrieved and passed to carta_envoy, this story's code writing is done.

This is an example of the file not existing:

The following tasks were done in order to complete this work:

• single_image is now True for VLASS coarse cubes when creating a workflow request in workflow_info.py
• For VLASS cubes in CARTA, the metadata wrester now gets the name of a file that exists (one of the SPW fits files). An example is VLASS2.1.cc.T10t02.J010200-013000.06.2048.v1.spw9.IQU.iter3.image.pbcor.tt0.subim.fits.