Suggested bug fix for pyclaw Issue #457 - Fix for capacity function weighting in 3D transverse solvers

This is potential bug fix for pyclaw issue #457 where the capacity function weighting was not properly applied to the transverse_waves = 21 and 22 solvers.  It should have no impact when capacity functions are not used. It should pass the proper dtdy and dtdz values from step3() to flux3() when capacity functions are used.