Add schema.org mapping (see #75 for details)
This PR provides:
I found a way to check if a helper function is defined.
In ckan/ckan#3042 an excpetion was added to the helper dictionary, so it's no longer possible to simply use
The SchemaOrg schema provides a number of methods that could be overriden by subclasses if they need a slightly different mapping to schema.org (e.g. from one of the DCAT AP standards). With those methods in place it should be relatively easy to implement a schema.org mapping for different standards.