Add flag to loader for whether to load chromosome or supercontigs, or both

[alancleary]

Without a flag it's just going to load whatever is in there. With a flag, you should provide a list of things you want it to pay attention to, i.e. chloroplasts. It there should be a finite set of one or more options for them to choose from, i.e. use a ArgParse choices parameter: https://docs.python.org/3/library/argparse.html#choices

[adf-ncgr]

It looks like someone did try to implement something along these lines, but at least with respect to the chado_loader it seems pretty broken. Not that anyone is likely to use the chado_loader for much of anything these days, but since I had to tussle with it briefly when dealing with some old arabidopsis datasets, I'll record a few observations that may be useful for later if we try to deal with it more properly:

• the use of nargs="+" here is problematic given that (I think) the current structure of the command line requires the chado/gff arg to follow, along with their specific options
• the customized EnvAction doesn't provide any obvious (to me) way of getting a list defined as this one requires
• the downstream consumer is trying to iterate over a string because of the previous point, leading to errors about not finding cvterms with name 'c' (from 'chromosome')