reformat collection/entity modules for clarity and consistency

- Moved COLLECTION_FIELDS and ENTITY_FIELDS to top of module.
- Better explanatory comments before blocks of functions.
- Renamed prepare_* to formprep_*.
- Renamed process_* to formpost_*.
- Renamed form prep/post functions in DDRLocalCollection and
  DDRLocalEntity for consistency.
- Moved utility functions below module functions and added
  explanatory comments.
module_(xml_)function; (collection|entity)spec -> module

- Moved duplicate code into module_(xml_)functions.  EAD/METS
  XML writing requires extra args that the display and forms
  prep/post didn't need.
- Renamed collectionspec and entityspec variables to something
  more explanatory.
- Explanatory comments.