Fixing subset in the DCPSE_slicer

src/DCPSE_op/DCPSE_op_subset_test.cpp

src/FiniteDifference/FD_Solver_test.cpp

src/FiniteDifference/FD_expressions.hpp

src/FiniteDifference/FD_op.hpp

src/FiniteDifference/FD_op_Tests.cpp

src/Operators/Vector/vector_dist_operators.hpp

src/Solvers/umfpack_solver.hpp