This function initializes the UDUNITS library. It is called by .First.lib when the package is loaded. Normally, the user does not need to call this function.
If path is non-NULL and not empty, then it specifies a units file containing initializing unit definitions; otherwise, the environment variable UDUNITS_PATH is checked and, if it exists and is not empty, then it is assumed to contain the pathname of the units file; otherwise, a compile-time default pathname is used.