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.