Adding slice + merging

src/DCPSE_op/DCPSE_Solver.hpp

src/DCPSE_op/DCPSE_copy/Dcpse.hpp

src/DCPSE_op/DCPSE_copy/SupportBuilder.hpp

src/DCPSE_op/DCPSE_op.hpp

src/DCPSE_op/DCPSE_op_test.cpp

src/DCPSE_op/DCPSE_op_test2.cpp

src/Operators/Vector/vector_dist_operators.hpp