Use OHDSI ddl defintions to build OMOP data internal data models and output

[PhilAppleby]

Background: One of the changes made when adding the "mapstream" function was to switch from using the internal CDM definitions based on Django to dynamic loading of json configuration in which lists of output fields, date fields, datetime fields were used - the aim being to allow data output to be handled via configuarion with no code changes required to add / modify data objects.

The next logical step is to generate the same json (python dictionary) from OHDSI-managed ddl definitions, removing the need for code upgrades on version changes and to cater for all OMOP data tables.

[PhilAppleby]

This work is currently in test