This adds loading of multiple yml files from various locations.
Namely, in order, with later files taking precendence over earlier ones
the following directories are scanned for any .yml file.
The order of the files within each directory is standard globbing order.

- $PACKAGE/etc/
- /etc/climix/
- <XDG directories>/climix, including
- ~/.config/climix/

Adds handling of negative parameters in templated indices,
ie a template like `tngt{TT}` can be used as `tngtm2` to
use a threshold of -2.

It also comes with a massive change in the handling of metadata based on python dataclasses.
After the fact, I discovered the attrs project and I might consider redoing this in attrs
to gain access to validators etc. But for now, it is good enough and should be revisited
in the future to include validation one way or another as well as a possible refactoring
of some of the standalone functions into classmethods.

* Move sliced_mode to output layer in `save` in main
* Store individual slices as they become available

* Add context manager approach
* Add commandline option to choose scheduler

# Conflicts:
#	climix/main.py

Fixes two bugs in SpellLength:

* Remove superfluous kwargs
* Add proper masking