Provide warning if beam size is larger than image

[AntoniaR]

When using images from alternative sources (e.g. online catalogues), the metadata may be in a slightly different format to that expected. Specifically, the restoring beam can be specified in different units. We use beam parameters in degrees (as output by casa) but, for instance, the TGSS catalogue specifies the beam parameters in arcsec. Thus, the sourcefinder thinks that the restoring beam is 3600 times larger than it is and meaning that the beam is larger than the image. This means that the sourcefinder fails giving the error:
RMS map masked; source finding skipped

Although it will be impossible to catch all instances of this, a warning that the restoring beam parameters are larger than the image would be helpful in these extreme cases.

[AntoniaR]

This is an ongoing wish list item but not essential for R7. Moving to the long term milestone.