Mltraq

* Moved plotting utils to the `cumulative` viz project

* Added `utils/plotting.py` module to facilitate the rendering of matplotlib visualizations

* Added Python and MLtraq version to experiment's metadata
* Added `utils/sysenv.py` to track environment stats (system, architecture, locale, time, ...)
* Added `utils/reproducibility.py`, providing a temporary seed context
* Added `utils/plotting.py`, providing a temporary plot context for a single axes
* Removed `utils/plot.py` due to lack of usage (and unlikely usage in the future)
* Improved `BaseOptions.get(...)` with `prefer` and `otherwise` parameters to make it more flexible

* Increased tests to `87%` coverage

* Using `Union[x,y]` instead of `x|y` for type definitions for increased Python 3.9 compatibility
* Added a read-only mode for `BunchStore`, preventing an unnecessary write check
* Added support for local files in `fetch` module

* Fixed a race condition in `BunchStore`
* Added support for Python 3.9
* Verified tests on latest versions of dependencies