Explore how to add descriptions and units to plant export data.

All of these will need to have a description and units too, similar to the input data and constants.
Not critical to include in this PR, but we'll need to address this before doing validation of the simulation outputs. I will look into this under https://github.com/ImperialCollegeLondon/ve_data_science/issues/441

_Originally posted by @arne-exe in https://github.com/ImperialCollegeLondon/virtual_ecosystem/pull/1816#discussion_r3946122350_

Begin by comparing plant export data with the existing input-data and constants metadata. Review issue 441, where this work is intended to continue, and determine how descriptions and units should be represented; done means plant exports carry both fields consistently for later output validation.

Written by the indexing model from the issue text.