Regsearch doesn't find SIA version 2 services.

[tomdonaldson]

The following query only finds services SIA version 1 services:

import pyvo
image_services = pyvo.regsearch(servicetype='sia')

This is because the servicetype='sia' constraint uses a SERVICE_TYPE_MAP that can only construct the standard ID for SIAv1 (ivo://ivoa.net/std/sia), but not the standard ID for SIAv2 (ivo://ivoa.net/std/SIA#query-2.0).

So far the pyvo code for searching SIAv1 and SIAv2 services treats the searches as 2 different types of things. This probably makes sense since the capabilities for v2 are quite expanded and even the basic part of the v1 search was changed. This means that in addition to being able to find SIAv2 services, regsearch() needs to be able to instantiate both v1 and v2 SIAService objects. (Note that the SIAv2 version of that class should be named differently, but that will be mentioned in a separate issue.)

While regsearch() should support explicitly finding one version or the other, it's less clear how it should handle the servicetype='image' case, which is currently just a synonym for the servicetype='sia'. Should a client searching for image services then have to look at which types of services it got back?