use introspection to find prepare_/process_/ead_/mets_

Many collection and entity fields contain structured data that
requires preparation before it can be used in a form, and post-
processing once it comes back from the form.
Many fields require special functions to insert their data into EAD
or METS XML.
These functions had to be explicitly linked to from COLLECTION_FIELDS
and ENTITY_FIELDS.

With this commit, functions with a field name and a prefix will
automatically be executed if they are present in
ddrlocal.models.collection or ddrlocal.models.entity.
Examples:
- prepare_persons
- process_topics
- ead_created, ead_lastmod
- mets_id
display preprocess functions for collections,entities

Automatically finds display_{fieldname} functions in
ddrlocal.models.collection and ddrlocal.models.entity and
executes them before sending to template context.
slight fix on entity.prepare_topics to accommodate raw and data

Has to work with semicolo-separated lines of raw subject headings
or with label/url dicts.