Keep ancillary variables

[aulemahal]

Is your feature request related to a problem? Please describe.
A catalog would list all the "primary" variables in its assets. If one of the assets contains ancillary data for the primary variable, so variables not listed in the catalog, those do not get included in the output. Coordinates are included and even grid mappings (#449), but not variables listed in the CF ancillary_variables attribute. This can include quality flags, for example.

Describe the solution you'd like
I'd like the variables listed in the ancillary_variablesattributes of the requested variables to be included too, given that they are in the same dataset.

Describe alternatives you've considered
Including them in the catalog. This yields larger catalogs and seems inelegant. Also, that means the user most know before hand what are the names of those extra variables, which might not be always easy.