Riemann solvers containing common blocks not thread safe

[rjleveque]

Most of the Euler Riemann solvers still have common blocks, which are not thread safe with OpenMP.

This showed up when testing amrclaw/tests/quadrants.

Some other Riemann solvers also have common blocks. Need to clean all of these up.

[ketch]

Almost all of the non-threadsafe common blocks were removed in #30. The only remaining one is the use of dxcom, dycom in the mapped grid solvers (which is non-threadsafe in AMRClaw only).

[ketch]

The only problem left (before we can close this issue) is the use of dxcom and dycom in the solvers for shallow water on the sphere. I'm not even sure they're being used correctly there, though.