Release 0.6.4 - meridional flux correction

This release includes the fix for the calculation of meridional flux component. See the correction note for details. The corrigendum on Neal et al 2022, GRL corresponds to this release.

The main change in the calculation can be found in this commit: 38d95f6

Please remove all the compiled fortran modules on your machine and git pull the newest version of fortran codes for installation (python setup.py develop).