Create new IamcDataFilter class

[danielhuppmann]

This PR is a preparation step for a new DataValidator processor, see IAMconsortium/common-definitions#104

Given the overlap with the already-existing RequiredData processor, the first step is to pull out a common underlying class to collect IAMC-format filters.

In the process, I've been debating with myself whether the DataValidator should support the measurand feature...

Any thoughts @phackstock?

[phackstock]

In the process, I've been debating with myself whether the DataValidator should support the measurand feature...

At some point it might be nice/necessary to do that. For a first implementation though I'd go for maximum simplicity. If the unit is not what we expect, the numbers will be wrong and it would be caught by the validator anyway.

[danielhuppmann]

Seems like upgrading to matplotlib 3.9.1.post1 does the trick...

[phackstock]

Thanks for the PR, couple of questions/comments below.

[danielhuppmann]

Thanks @phackstock for the thorough review. I played around a bit, but it always ended up being way more complicated than it should be, so I concluded that a little bit of duplication is better than a very complex piece of code...

[phackstock]

Thanks for the re-write @danielhuppmann, fully agree with "a little bit of duplication is better than a very complex piece of code" 👍
One small in line comment but good to merge from my side.