Remove common blocks from Riemann solvers where possible.

This pull request eliminates all common blocks for Roe averages, and almost all common blocks for dxcom, dycom.  It leaves common blocks that contain scalars -- for now -- since they are threadsafe.  It also leaves dxcom, dycom in mapped grid solvers, since we don't yet have another mechanism for passing those in.

PyClaw tests all pass.  Please test these with any other available tests from other packages.