Fixed wrong class name
Necessary preamble.
This function should be pure virtual.
Need to define interface for the init function.
Need the reactant species index in the constructor
First draft of GasRecombinationCatalyticWall

This class handles the case of a catalytic wall where a gas is
recombining, e.g. N+N -> N_2. We are making an assumption that
only these species are involved in the reaction so that the product
mass fluxes can be computed as minus the reactant fluxes.
Build new catalytic wall classes.