Geometry computations in domain_making

2 items to fix (REK):
- Gauss : symmetrisation of number of points in southern hemisphere to be removed, half sphere is enough in namelists
- LAM: default size of I zone is calculated in order to have an I zone of 20km; that has no scientific justification.
   Let's keep 8 and 16 for resp. 2.5km or coarser and 1.3km, and no default otherwise