Add check on guide image size

[jeanconn]

Add check on guide image size

If we're adding API to support selecting other image sizes

sot/proseco#344

this should probably be checked in sparkles (which will let us remove such checks from starcheck). Not sure if the defaults should be captured in a config in proseco.

[taldcroft]

In sot/proseco#344 have made a new GuideTable.get_img_size() method that encapsulates the default and returns the "correct" answer for the catalog given the number of fids.

[jeanconn]

Great idea! So we can get the size to double-check the catalog in sparkles and if we switch to 8x8 for ORs we only have one place to make a change.