Errors in river estimation geometry

[AliceHarang]

After testing the river estimation tools on a wider pool of configurations, some errors arise.
These are here shown on the Porangahau river (ocean rid: 8034414).

• River bathymetry points location in the center of bends:
  
  (the "river_polygon" is well located, so are the "final widths" but the "river bathymetry" points are in the center of the bend)
• Very variable extent of the river_polygon in the lagoon/mooth area:
  
  (These oscillations of the river_polygon (grid) are observed along the "lagoon area". For this case, we also observe higher ground dem (blue background map) in the water (~0.5m) than on the banks (~-0.5m)).
• Island or multi-channels:
  
  There is an island in the middle of the river (and then two channels), and the river_polygon jump from on to the other.