RF+DOC: refactor Analyze-type header conversion

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

Document the ``as_analyze_map`` interface in docstring.

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

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

Remove unused ``_set_format_specifics`` from freesurfer/mghformat