Merge pyicon_dwd with master

Merge changes from branch pyicon_dwd into master to allow pyicon/quickplots to be used with ICON-NWP.

The branch pyicon_dwd was created by Gergely Bölöni who adopted the pyicon/quickplots so that also simulations performed with ICON-NWP can be visualized by the quickplots. Some additional diagnostics were added. Minor changes in other files than qp_driver.py were necessary for a successful inclusion.