Improve `lfcoords`

[casadoj]

Ideas for improvements in the lfcoords tool:

• Remove the -r flag for reservoirs, as it was discovered that reservoirs DO NOT need to be located one pixel downstream of their actual location.
• Add a flag to overwrite results. By default, results would not be overwritten, and the functions will skip points for which results are already available.
• Create the MERIT mosaic on the fly.