develop method to retrieve checks from metacat

[jeanetteclark]

Currently checks are retrieved from the file system, which means every new version of a check needs a new file. We should retrieve these from metacat instead, by the check id. This would mean that new versions of checks released in metadig-checks would get inserted into metacat, so in addition to new methods here for retrieving checks we would also need to write some methods in to the build process of metadig-checks that would do that publish step