downstream support for get_data implementation

[kecnry]

This PR implements mask_subset in the private _get_data method which applies any generic subset as a mask. spectral_subset is then a special case of this mask_subset where type-checks are done to ensure its spectral. Downstream apps (such as lcviz - see this PR for required changes there if this is adopted) can then write their own get_data method which does its own type-checks, if necessary, and passes any type of subset to mask_subset (as long as it doesn't also pass spectral_subset).