Erik. New boundary condition BCOutflowSupersonic implemented
Erik. 1D test with two colliding steel plates running.
Erik. Steel vs Steel collision test reference version.

1) Bug fixes in GasModel
2) Implemented timed snapshots
3) Bug fix in RiemannSolver
4) kernel changed to store pointer to GasModel
Erik. Pb vs Pb collision reference code.
Erik. GasModel class made pure virtual as planned. PerfectGasModel implemented in child class.
Erik. Added new boundary condition for SuperSonicInflow.
Erik. Runable code. Pb vs Pb collision 1D test. Fixed to compile with g++.
Merge branch 'erik.collision.test'