Improve quantiles #278, add statistics category #277

[m-mohr]

First draft for #277 and #278.

• Added "math > statistics" category to processes for a more fine-grained categorization add "statistics" category to quantiles #277
• Remove "nearly" from quantiles process q param in quantiles #278
• Make clear that the probabilities array is inclusive
• Clarify "intervals to calculate" => "the number of intervals to calculate"

Still to do (open for suggestions):

• Wording around probabilities. This is general statistical language and is potentially useful to define, but @soxofann raises that this is not really fitting here.
• Check whether we also want to implement quantiles: Unify probabilities and q #293 (deprecate parameter probabilities, unify to parameter q) -> separate PR

[soxofaan]

Related: while looking up example definitions and descriptions I stumbled on this:
https://blogs.sas.com/content/iml/2017/05/24/definitions-sample-quantiles.html referencing https://www.amherst.edu/media/view/129116/original/Sample+Quantiles.pdf

there are nine definitions of sample quantiles that commonly appear in statistical software packages.

If we want consistency across backends, we probably have to pick which flavor of "sample quantiles" we want.

at the moment I have no idea which flavor that's currently used in VITO/EODC backends for example

[m-mohr]

Ouch, that's painful. So we should probably asses which implementations we currently have, otherwise a useful process description is impossible. Doesn't help if we enforce a type that's not implemented anywhere. Anyway, this is something for a separate issue and I'll continue with these changes anyway.

Edit: New issue is #296