New tool catchstats and updates to ncextract tool

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• New tool called catchstats used to compute catchment statistics. The tool can be used either from the command prompt or imported in a Python script:
  from lisfloodutilities.catchstats import catchment_statistics
• Updates to ncextract tool, that now allows to read both NetCDF and GRIB files. The tool is now able to read input files whose coordinates are not named 'lat' or 'lon', as it was hard coded up until now. This was a problem because files in the Climate Data Store use the names 'latitude' and 'longitude'. Any coordinate name that starts with 'x', 'y', 'lat' or 'lon' will be recognised now.
  Code is now split in three functions: read_inputmaps, read_points and extract_timeseries. In this way, the function extract_timeseries can be imported an used in the pipeline of any Python script.
  from lisfloodutilities.ncextract import extract_timeseries
• Updates to the README.md to improve the readability of some documentation.