Throwing an exception when a state variable is being set to NaN transforms some
reactor network integration errors from unrecoverable errors into recoverable
errors.
This makes it easier to set the composition of a phase based on the composition
of another phase with a different set of species. The threshold argument allows
species with negligible concentrations to be skipped.
Deprecate the unused getMoleFractionsByName function that didn't return a
value.
