Modify select behavior when only some of the requested data exists

[jpober]

Currently if one tries to select data that doesn't exist, it errors. This also happens (at least in some methods, but maybe not mir or miriad? @kartographer?) if only some of the data you ask for doesn't exist. For example, if you ask for baseline (0,1) and baseline (0,2) but only baseline (0,1) exists, you will get an error. It would be helpful to have it return the data that does exist and warn instead that not all the requested data is present. Open to this being default behavior or a keyword.