Early implementation of `io` subpackage interface

Early implementation of `read` module as principal interface to `io`
subpackage. File-type detection code requires further review and
revision.

Add UTF-8 encoding to all source-file headers.
Further work to develop `detect_filetype()`

Further work to develop `detect_filetype()` based on the premise of a
maximum of two file extensions.
Reset `detect_filetype()` for re-implementation

Reset `detect_filetype()` in order to re-implement function more
robustly and simply.
Initial work on function notes

Initial work on function notes for `detect_filetype()`, to define valid
filepaths.
Re-implement `detect_filetype_exts()`

Re-implement `detect_filetype_exts()` and supporting function
`clean_file_ext()`. Still to add in functionality for zip files.