amf_variables: Get FP (Flux-Processing) Standard Variable List
Description
This function obtains the latest AmeriFlux FP (Flux-Processing)
standard variable list. FP standard defines the variable names and units used
for continuously sampled data within the AmeriFlux. Also see AmeriFlux Data
Variables page https://ameriflux.lbl.gov/data/aboutdata/data-variables/
for details.