Fixing interpolation test

src/FiniteDifference/Derivative.hpp

src/FiniteDifference/FDScheme.hpp

src/FiniteDifference/Laplacian.hpp

src/Solvers/petsc_solver.hpp

src/Vector/Vector_petsc.hpp

src/interpolation/interpolation.hpp

src/interpolation/interpolation_unit_tests.hpp