Revamped lfcoords

[casadoj]

I have revised completely the function lfcoords. Improvements:

1. It only needs as raster inputs the flow direction maps. It doesn't need any more the upstream area maps, as it computes them under the hood.
2. It allows to only do one of the steps: locate the points in MERIT (fine grid) or locate the points in LISFLOOD (coarse grid).
3. I have revised the location of stations in the LISFLOOD grid, which used to cause errors in small catchments. The previous setup was only based on shape similarity with the polygons in the MERIT grid. The new implementation delineates the LISFLOOD catchments using a combination of shape similarity and catchment area error compared with the MERIT polygon.
4. I have refactored the whole tool to reduce memory limits. Among others, data is only loaded when needed and memory is released once it's been used.