Stiff chemistry solver validation test cases

gri    : GRI-Mech 3.0. CH4 combustion, 53 species, 325 reactions

h2     : H2 combustion, 10 species, 27 reactions

nc7h16 : n-Heptane combustion, 544 species, 2446 reactions

ic8h18 : iso-Octane combustion, 874 species, 3796 reactions

Results interpreted in 'validation' sub-folder, where OpenFOAM results
are compared against those predicted by Senkin (CHEMKIN II)

