Merge branch '239-implement-prestress-for-eg' into 'feature'

mpp

src/elasticity/MainElasticity.cpp

src/elasticity/assemble/DGElasticity.cpp

src/elasticity/assemble/DGElasticity.hpp

src/elasticity/assemble/EGElasticity.cpp

src/elasticity/assemble/EGElasticity.hpp

src/elasticity/assemble/IElasticity.hpp

src/elasticity/solvers/IterativePressureSolver.cpp

src/elasticity/solvers/StaticSolver.cpp

test/elasticity/CMakeLists.txt

test/elasticity/TestPrestress.cpp