Update schema mappings

This is meant to address a number of inaccuracies in the mappings between the POD schema and DCAT as well as CKAN (eg as seen in API calls) 

This does not yet list CKAN mappings that would only exist as CKAN extras. 

This doesn't attempt to solve all incompatibilities between POD and DCAT, but does suggest recommendations/deprecations in the case of `dcat:downloadURL` and `dcat:mediaType` 

A few unaddressed issues worth noting (and which should probably be separately ticketed) are: 

* In DCAT `contactPoint` is required to be a URL to a vCard, so that doesn't match the usage in POD. 
* `license` doesn't map to anything in DCAT for the dataset. In DCAT licenses are only for the whole catalog or for each distribution while in POD they are meant to refer to the dataset.