unit.conversions is basically a 'look-up' for unit conversion methods. Each element of
the list is another list. These lists are each individual conversion methods comprising four
elements: to and from, character vectors given the unit ids and alias of the unit
types that can be converted using the method; conversion, a function for the associated
conversion method; and (possibly) tag, a more detailed description of the conversion
intended for use in documentation.
Other ref... are sets of constants or reference information stored as lists. ref.chem
contains atomic weights of some elements and molecular weights of some species. ref.petrol and
ref.diesel contain default properties for typical fuels.