Generalizing material properties to use in chemical_reactions module

Miaomiao Jin (Mike Short's student) has generalized two kernels in the chemical_reactions module to accept variable strings for material properties, instead of fixed material properties called "diffusivity" and "porosity." This allows users to use the same kernels multiple times, with multiple material properties. For example, let's say we had a material with two diffusing species, with properties called "diffusivity_A" and "diffusivity_B". In the old way, we would have had to construct two material files, each with a material property called "diffusivity," to make this work. Now we don't have to.