Restructuring of FluxnetSimulationsExt to support any FLUXNET site

[ThanhNguyen428]

Purpose

FluxnetSimulationsExt currently only supports the simulation of Flux sites US-Ha1, US-MOz, US-Na1, and US-Var. This PR allows users to simulate any site from the FLUXNET2015 database, using simulation parameters (such as soil, hydraulics, etc.) gathered from maps and from a site's plant functional type (PFT).

To-do

Content

• Created functions to get spatially-varying g0 (medlyn model intercept), z_top (canopy height), and Pft() (ClimaLand's internal PFT type)
• Wrote a retrieval function for a given generic site's infrastructure information
• Wrote a retrieval function for a given generic site's geospatial and PFT-based parameters
• Edited run_fluxnet.jl to support new "generic" functionality alongside previously-mentioned sites

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• I have read and checked the items on the review checklist.