Define the method of how to check for the type of vertical coordinates

[petrabaumann]

cfgrib derives the type of vertical coordinates from the parameter attribute GRIB2_typeOfLevel, and creates a dimension coordinate with the same name.

Thus, either the name of the vertical dimension, the name of the associated dimension coordinate, or the attribute GRIB2_typeOfLevel could be assessed to check the vertical coordinate type.

In the current code, a mixture pof methods is used (e.g. destagger.py, and vertical_reduction.py).