Second phase of outputting static data files under a repository tree.

 * pyrseas/database.py (Database._link_refs): Pass datacopy config to
   schema link_refs; also, pass dictionary of dictionaries instead of
   individual ones.  (Database.to_map): Do not set opts.datacopy.
   (Database.diff_map): Call schemas.data_import of
   tables.data_import.
 * pyrseas/dbobject/__init__.py (DbObject.extern_filename): Add file
   extension argument, default 'yaml' to also support 'data'
   extension.
 * pyrseas/dbobject/schema.py (Schema.to_map): Handle datacopy by
   calling tables data_export.  (Schema.data_import): New method to
   handle datacopy in other direction.  (SchemaDict.link_refs):
   Refactor by accepting dictionary of dicts; also process new
   datacopy argument.  (SchemaDict.data_import): New method to iterate
   over schemas/tables to import.
 * pyrseas/dbobject/table.py (Table.data_export): New method to copy
   table data out to a file.  (Table.data_import): Add dirpath
   argument and expect table in specific path.  (ClassDict.link_refs):
   Refactor processing wih a local link_one function; remove datacopy
   handling.  (ClassDict): Removed data_export and data_import
   methods.
 * pyrseas/testutils.py (DatabaseToMapTestCase.to_map): For
   multiple_files and datacopy, merge metadata and data repo info.
   (DatabaseToMapTestCase.yaml_load): Read file from metadata_path.
   (InputMapToSqlTestCase.to_sql): For datacopy, merge data_path info.
 * tests/dbobject/test_static.py: Refactor tests to output/expect
   files in data_path config location.