FCI: yup and y+1

[dschwoerer]

Requiring both yup and y+1 tripples memory consumption, as the non fci y boundaries are not to be used, and it is not allowed to use yup without shifting y by one, or using y+1 on the main field. This causes various issues (see e.g. #2324 )

It would be better to not require y+1 on the yup fields. Needs fixing of the DDY indexing code, which is slightly non straight-forward to understand.