Update traits data structures

[davidorme]

Description

This PR implements the updated trait data structures requested in #298. It update tests and things using the trait data structures.

It forms part of a sequence re-implementing the functionality set implemented in #296 but on less shaky structural grounds and in smaller more easily reviewed sections.

Fixes #298 (issue)

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[j-emberton]

Would it make sense to make Flora inherit StemTraits, or have both inherit from an ABC, so that this list of traits is only defined in one place?

[davidorme]

Ha. I did look at all sorts of things here - Protocols and ABC and inheritance, but these classes are basically intended to deal with two different constructor signatures, even if they share nearly all the attributes. That seems to make any kind of inheritance more finicky than just duplicating stuff. I may have missed it - I'd love a way to share that set of attributes cleanly across the two PFT traits, Flora and Stem Traits.

Right now though, I'd like to define that common API on the attributes and get the next PR in to close out the functionality hanging in #296 - and if the API looks good, I'm all for circling back to improve the implementation later.

[j-emberton]

Happy to approve to keep the demography work stream moving. I'm wary of accumulating technical debt (i.e. not coming up with an improved implementation of the stem traits that avoids duplication) but appreciate we need to get the remaining stuff in as well.