Rutines for converting runoff data into river data used by e.g hycom.

Environment variables:
ERA40_PATH : points to location of ERA40 data with runoff fields
TRIP_PATH  : points to location of TRIP data (can be found under Data in this dir)

Routines:
trip_paths   - mostly diagnostic routine to calculate largest river catchment basins 
               and grid cells with largest river flux
trip_weights - Calculates the mapping from a runoff grid (here ERA40) to TRIP grid. Must be run
               before trip_flow
trip_flow    - Calculates the transport of runoff water from its initial appearance through rivers
               and towards the sea. Ues ERA40 runoff data.