Add a unified interface for two register VdH-type RK schemes.
Add a unified interface for two register VdH-type RK schemes.

Using this we simplify the existing RK4(3)5[2R+]C scheme and
add a new RK3(2)4[2R+]C scheme.

tjcorona/PyFR,BrianVermeire/PyFR,tjcorona/PyFR,tjcorona/PyFR,Aerojspark/PyFR,iyer-arvind/PyFR