Only treat activation energies as a special case, rather than all molar
energies. Units of activation energy can be set either explicitly or by setting
units for energy and quantity. Only the case where activation energies are given
as temperatures needs to be specified explicitly.
Allow setting of default energy units, which allows calories to be used.
Also add dyn/cm^2 as an option for pressure units.
Introduces a new, more natural notation for writing unit strings,
for use in YAML input files. Unlike 'toSI', conversions are checked for
dimensional consistency.
