fixed _files_generator to recognize .zarr files

gather_files now accepts ncores as an argument to enable parallel fet…

…ching of file paths, improving extraction speed for large directories

gather_files returns empty data frames if target doesn't exist

Environment change


- distributions are built with most recent pip and setuptools

Provide GroupColumnByN and file gathering utility

- `GroupColumnByN` allows to group rows in a column by size
- The `gather` function provides a fast means of collecting files based
on a naming convention

Zero version containing just the package skeleton