Querying Solar System Object Image Search

It would be nice to have a new module to access the [Solar System Object Image Search](http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/ssois/index.html) service provided by the CADC. Here is some API documentation (https://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/ssois/apidoc.html).

Start with the linked Solar System Object Image Search API documentation and determine the service operations the new astroquery module must expose. Done means adding a module that can access the CADC service described there; the issue does not name existing files or tests to run.

Written by the indexing model from the issue text.