The 'pykrige', 'pyfftw' and 'basemap' requirements are no longer optional...

The 'pykrige', 'pyfftw' and 'basemap' requirements are no longer optional since they are easily installable from conda-forge now. Updated requirements and installation instructions. Updated version info.

Signed-off-by: Daniel Scheffler <danschef@gfz-potsdam.de>