Resolve "Grib Support for ICON-D2 open data: Wrong data in single level fields"

Fixes #328 (closed).

The grib reader reads non-surface level type 2D fields also as single level fields, to be able to support all 2D level types. It does so, by setting the level to 0 for each variable. The offset in the file to the data-grid however, was still mapped to the old level, resulting in wrong data reads. This MR fixes it, by also remapping the data offsets for each data file of the variable.