Small code cleanups

* Use _chunk_write inside _chunk_read as well as in _chunk_read_ftp_resume
* Remove unused parameters for _chunk_write, _chunk_read, _fetch_file
Remove unnecessary overwrite code duplication

Removing old datasets and overwriting is already done better in data_path(),
which calls _fetch_file().
Make _fetch_file behave more like old code

The parameters of _fetch_file (which was adapted from NILS) are now identical
to the ones from the function _download_status being replaced (apart from an
added resume parameter which is there only because of the new download resume
functionality).

Other small things:
* Documentation of _fetch_file is now also more accurate.
* Remove unnecassary returns from _fetch_file
* Remove unnecessary directory creation code from _fetch_file
Use new download with resuming code for all example dataset downloads

Also removes the _download_status function which is now unused.
Remove unused variable assignment and related module
Simplify download messages