dt extraction limited to DATABASE_NODOUT

[mlp6]

The current extract_dt() function in create_res_sim only looks for the DATABASE_NODOUT keywork field to find the time step increment, but some models only have DATABASE_BINARY_D3PLOT defined. Would be good to have this be more robust. Either look for both and choose one by default, have the user specify the string to look for, or maybe allow the user to just specify it. ??