Removed default values in each method of diffusivity.
Removed default values in each method of diffusivity.


Former-commit-id: 970d7a101acc526367f21c17f1b8f2c256b40c71
Former-commit-id: 21a151f1ecdf71afe391e7c6ba75cbdf8694c487
stadelmanma/OpenPNM,TomTranter/OpenPNM,amdouglas/OpenPNM,amdouglas/OpenPNM,PMEAL/OpenPNM