RF+DOC: refactor Analyze-type header conversion

Refactor header conversion routine to use basic conversions always, and then
mapping if available.

Document the ``as_analyze_map`` interface in docstring.

Add ``as_analyze_map`` tests and as API documentation.

Rename ``_set_format_specifics`` to ``_after_mapping_cleanup`` to make clear
that the routine is to set needed default after creating a header from a
mapping.

Remove unused ``_set_format_specifics`` from freesurfer.