Add new criteria for API search

[john-shepherdson]

Markus wrote: "Christian Ohmann (and Sergey Goryanin) contacted me in September to discuss the technical implementation for their MDR + CESSDA search demo. They wanted to directly use SearchAPI in their search but they needed to search by Sampling Procedure, Mode of Collection and/or Time Method for the mappings they wanted.
I think they ended up harvesting OAI-PMH and setting up their own copy of the index after I showed them where to find the Elasticsearch settings and mappings used in CDC since they were in a rush to get the demo done before the project ended at the end of September. Demo won't be expanded upon unless they get more funding."

[john-shepherdson]

At present, the API search criteria mirror those available via the CDC UI. Logically, any new criteria should be added to both, not just the API. Therefore a discussion is needed in the CDC User Group as to whether or not Sampling Procedure, Mode of Collection and/or Time Method should be added to the existing set of filters in the UI, and if so, what the priority is.