Adding Petsc Equation Structures

src/DCPSE_op/DCPSE_op_test.cpp

src/DCPSE_op/DCPSE_op_test2.cpp

src/DCPSE_op/DCPSE_op_test3d.cpp

src/DCPSE_op/EqnsStructEigen.hpp

src/DCPSE_op/EqnsStructPetsc.hpp