This function classifies a fire danger index based on user
defined danger thresholds. If index
is one of GEFF's indices (fwi,
ffmc, dmc, dc, isi, bui), the user does not need to specify thresholds and
labels. Default thresholds and labels are those defined by JRC for Europe:
https://effis.jrc.ec.europa.eu/about-effis/technical-background/fire-danger-forecast/
For instance if index = "fwi", default thresholds and labels are:
very Low = 0-5.2, low = 5.2-11.2, moderate = 11.2-21.3, high = 21.3-38.0,
very high = 38-50.0, extreme = > 50.
However, if threshold != NULL, the default values will be overwritten by the
custom values.