Update plants model to use pyrealm flora definition

[davidorme]

The implementation of the plant functional types and the Flora object in the Plants Model are cut down placeholders for the more complex definition that is now implemented in pyrealm (as of 2.0.0-rc.1). We need to:

• Switch the Flora implementation over to pyrealm

• Think about how to configure the PFT definitions.

  This is actually a bit tricky: these are currently part of the TOML model configuration, but that is increasingly unpleasant for large numbers of PFTs and the set of traits expanded beyond the current placeholder of name and max height. The cleanest way is to provide a CSV dataframe, which pyrealm already supports, and add a PFT file path to the configuration.