Remove declare_var and update in sum

fenics_optim/convex_function/base_convex_function.py

fenics_optim/convex_function/epigraph.py

fenics_optim/convex_function/infconvolution.py

fenics_optim/convex_function/marginal.py

fenics_optim/convex_function/perspective.py

fenics_optim/convex_function/sum.py

tests/test_convex_functions.py