New initialize method for Trilinos AMG preconditioner with Epetra_RowMatrix

Epetra uses long long and does not provide a typdef but I think this is wrong, it should be int not long long int here. In the else, maybe you could use unsigned long long int. Trilinos returns a long long int but to be consistent with global_dof_index, we usually return an unsigned long long int (but then you will need to return an unsigned int for n_global_row and global_length).