Adds "fraction" (a dimensionless unit, for fraction excreted) and "mcg"
(an alias of "ug", the spelling clinical data commonly use) to the
units package. units::install_unit() changes the udunits database of the
R session, so these units are also available to every other package that uses
units once PKNCA is loaded. Installing a unit again with the same
definition does nothing, and "mcg" is not installed when udunits already
defines it.