Vectors: solution and current_local_solution

When 'PetscVector< T >::init()' (or 'NumericVector< T >::init()') is called with an integer second argument n_local different from the first argument N, then the vector is distributed with global size N and with n_local elements local to the process. If n_local is not provided or is the same as N, then a full separate vector of size N is created on each process.

System::solution uses the distributed form. This form seems suitable for performing multiplication operations with matrices of type 'SparseMatrix< Number >'.

System::current_local_solution, 'TransientSystem< Base >::old_local_solution', and 'TransientSystem< Base >::older_local_solution' use the full non-distributed form. Although the size is the full N, only the entries corresponding to process's local dofs and the ghost dofs it needs are non-zero. This form seems suitable for use in interpolating values in elements.