Explore structures to synchronise trait details across the two PFT traits, Flora and Stem Traits.

[davidorme]

@j-emberton commented on #300 that the definitions of Flora and StemTraits are repetitious

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 something - I'd love a way to share that set of attributes cleanly across the two PFT traits, Flora and Stem Traits. The #300 PR was merged to establish that common API on the attributes and get the next PR in to close out the functionality hanging in #296.

This issue is to remind us to circle back to look at how to improve the implementation later.