Merge branch 'make-petsc-optional' into 'master'

CMakeLists.txt

src/Common/GmshFem.cpp

src/Dofs/DofsValueVector.cpp

src/Problem/Formulation.cpp

src/System/Matrix.cpp

src/System/Matrix.h

src/System/MatrixCOO.cpp

src/System/MatrixCRS.cpp

src/System/PetscInterface.h

src/System/Solver.cpp

src/System/Solver.h

src/System/Vector.cpp

src/System/Vector.h