Major updates to data model documentation and api spec based on discussions this week:

* restructure the apc interchange format to be more general in handling identifiers, to use license_ref and to be better structured for handling multiple sources of payment from multiple funds
* break out the storage data model into Institutional Records and APC Records, the latter being a merge of 1 or more of the former and useful for reporting
* provide explicit mapping for apc spreadsheet to data model
* update CRUD api with more examples of on-the-wire data and add support for local ids
* add list of funder reporting requirements