Fix data flags var name generation

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes a issue raised in private communication by @RondeauG
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGES.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

Changes how the data flags variable names are generated.

In the previous version, every kwargs was sent to str2pint. We were relying on a try: except to catch those kwargs that weren't quantities. This had the caveat of requiring an explicit list of possible errors. @RondeauG had a case where op='>=' was triggering a ValueError, which wasn't listed.

I changed this "implicit" parsing to an "explicit" one:

• Data flags declare the variable name as a templated string.
• Parameters are iterated through and handled according to their "InputKind"
  • "Quantified" inputs are handled as before, but all others are passed as-is.

This required:

• Changing the registering decorator to accept a templated string argument
• Correctly annotating the inputs, which lead to adding missing thresholds to the declared units

Does this PR introduce a breaking change?

Yes, I made the arbitrary choice of changing how we stringify the minus sign:

Input thresh : -5.1 mm
Template : "values_greater_{thresh}"
Before : "values_greater__minus_5point1"
This PR: "values_greater_minus5point1"

However, I don't think this will affect many projects.

Other information:

@RondeauG , could you try this new branch with your code ? (That's why I made you a reviewer)

[Zeitsperre]

LGTM. Nice to see existing functions being used to handle signatures.

[Zeitsperre]

Why are parentheses needed here?

[aulemahal]

I added a decorator level to the function: @register_methods() actually returns the real decorator, using the default value (None, thus func.__name__) as variable name.

[Zeitsperre]

I don't quite understand, but I'll give it a look on Monday with fresher eyes.