new &inflow options

[matthipsey]

need to add in

• subm_elev (as submerged inflow entry elevation is currently still read in via strmbd_drag)
• max_travel_time (since during hot-start and data-assimilation runs, we need to ensure no inflow parcels are "lost" during stop and restart. This option will force inflow parcels to be inserted once time is maxed out.

[rqthomas]

@matthipsey @casper-boon looks like the first bullet point has been addressed (though it should be subm_height). What about the max_travel_time option? Can it go into Version 3.9?