Enable astroquery.ska prototype to support multiple data formats

Now that we have a basic version of the astroquery.srcnet (https://gitlab.com/ska-telescope/src/src-service-apis/ska-src-astroquery) , it could be expanded to define extra functionality navigating datalink.

This could be done by creating a factory to create the formats:

astroquery.ska.formats.formatFactory

Once this is done, users can create a specific instantiation of a class per data type, for example

import astroquery.ska.formats.formatFactory as SKAFormatFactory

import.astroquery.ska.formats.*

if(SKAFormatFactory.isCube(<identifier>))

      cube = SKAFormatFactory.getCube(<identifier>)

This class (cube as one example cube, image, spectra, visibility, etc) will have some features provided in the interface for this particular data type and described in the data link, for example:

subcube = cube.SodaQuery(circle=12 34 0.5, range=None, polygon=None, band=None, **kwargs)

(which could invoke internally pyvo.sodaquery)

Once one format is added as an example, new methods could be added later for cube or other formats)

Important: The code signature suggested is just to express the idea so it could be changed if there are implementation reasons that justify it