single representation of database structure

[rwhaling]

the current structure of metadata_types, metadata_hierarchy, metadata_fields, dates back to the very earliest Philo4 proof-of-concept, and is starting to show it's age. metadata_hierarchy and metadata_fields both could and should be generated dynamically from metadata_types. We also need a standard loader function for adding/modifying these at load time, to prevent the need for custom postFilters to do it now, as in the encyclopedie and perseus.