Reads geospatial data from 'the nature conservancy' on ecoregions,
major habitat types (MHT, or biome types) and biogeographic realms.
Value
Returns magpie object with a share of each spatial unit belonging to
a biogeographic realm and major habitat type.
Examples
if (FALSE) { # \dontrun{
readSource("TNC2019", convert = "onlycorrect")
} # }