Fix bug in Trilinos SparseMatrix::add/copy_from

We assume that the pointers to a Trilinos sparse matrix remain valid when a matrix
is copied from another one or added and the two matrices share the same sparsity pattern.
This is used in step-32. However, the fix introduced in 8d64256 could not ensure this
(while fixing a few other cases). The check if the two sparsity patterns point to the same
memory is not good (we can have the same sparsity pattern memory but different addresses).

Therefore, the add and copy_from methods need to be more careful to not set up a new
matrix structure when this is not necessary. In particular, the two methods now check
whether the column indices are the same in any case but without requiring calling
methods that change the memory layout.