Add VO methods to the base classes

[bsipocz]

Currently, we have 2 modules using pyvo, with at least one more in the works. So it's probably the right time to think about a common API and add VO related methods to the base class and/or template, e.g. query_tap, or query_sia, maybe even some properties. If the API would be sensible for the ESA modules, that rely on TapPlus, it would be even better (while I expect those modules could not really use the baseclass methods as is, but maybe this would help them to move to pyvo, too).

cc @keflavich @andamian @jespinosaar

[jespinosaar]

cc @esdc-esac-esa-int