Change to max time step limit use and outputting

`nsteps` (aka `iter`) is used as a maximum to the time between re-interpolating the velocity fields in time (keeping in mind that the fields are also updated whenever a drifter passes a grid cell wall). This had not been implemented quite this way before, though it was similar. Also, this parameter has been decoupled from when the drifter tracks are actually sampled (and previously, sampling the tracks affected the tracks). A new parameter, `N`, is used for how often to sample the drifter tracks, which is uniform across all drifters, and it does not affect the drifter tracks.