Z-R Conversions

Module zr takes care of transforming reflectivity into rainfall rates and vice versa

z_to_r	Conversion from reflectivities to rain rates.
r_to_z	Calculates reflectivity from rain rates using
z_to_r_enhanced	Calculates rainrates from radar reflectivities using the enhanced three-part Z-R-relationship used by the DWD (as of 2009)