Begin implementation of the isentropic 1d case.

src/language/modules/VariationalSchemeModule.cpp

src/scheme/CMakeLists.txt

src/scheme/IBoundaryConditionDescriptor.hpp

src/scheme/Isentropic1DExact.cpp

src/scheme/Isentropic1DExact.hpp

...cheme/Isentropic1DVelocityBoundaryConditionDescriptor.hpp

src/scheme/Isentropic1dTest.hpp

src/scheme/VariationalSolver.cpp