Refactor field parsing to allow value preprocessing about structominer HOT 1 CLOSED

Turn the parse method into a pipeline shared (not overridden) by all subclasses. This pipeline calls in order:

• (maybe a pre-super could be useful? what kind of parameters would it take?)
• super(self.__class__, self)._parse(...) starts the super chain from the instance's class's parent
  • for this to work, _parse must recursively call super when not provided with a value to parse
• self._pre_process with the value from the super _parse
• self._parse with the value from _pre_process
• self._post_process with the value from _parse

Then each class in the field hierarchy defines its own _parse. Decorators are used to define chains of pre- and post-processing functions, and the parser decorator is used to add to or replace the _parse chain.

from structominer.