MNT : made functions 'private'

Added docs and prefixed array uint casting functions
with `_`.
MNT : rename file_locker -> FileLocker
MNT : fn -> filename

 - changed fn -> filename in signature
 - added filename property
MNT : add doc to discourage enthusiastic linters
DOC : Added class-level docstring

 - also corrected default-value of as_grey to False instead of None