Enabling reach routing at headwater HRUs

[nmizukami]

Currently, reach routing in a headwater HRU is not performed because lateral flow (BASIN_QR) is always added at the bottom of reach (nothing is coming from the top of the headwater reach).

This PR provides an option to add lateral flow at the top of reach in the headwater.

New control variable is hw_drain_point (integer)-- 1-> add top of reach, 2-> add bottom of reach. Default is 2 as currently done.