Make ICON data UGRID-compatible (on-the-fly)

[schlunma]

In order to use more complex regridding features for data on unstructured grids (e.g., using iris-esmf-regrid), these unstructured grids need to follow the UGRID conventions.

Currently, ICON data does not do that. However, using iris.experimental.ugrid it is possible to make ICON UGRID-compatible with a few lines of code.

I will implement this for v2.7.

[schlunma]

I successfully managed to convert ICON to UGRID (see #1664), but there are some issues left (out of our hand mostly) that need to be resolved before merging this (I list them in the associated PR).