Merge branch 'ICON-support-V44' into develop (PR #221)

Merge branch 'ICON-support-V44' into develop (PR #221)

This merge adds support for ICON in the WPS.

The ICON grib2 data set is now able to be ingested by the ungrib program.
Information about the ICON data set can be found here:
https://www.dwd.de/DWD/forschung/nwv/fepub/icon_database_main.pdf

 1. Two Vtable files (pressure and model levels) have been created with the
    fields required by the WRF model. Minor changes have been adapted compared
    to PR #154.

 2. Modifications to the ungrib source code: Adding support for level code 150
    in rd_grid2.F because model levels of ICON are on generalized vertical
    height coordinates. The general vertical height coordinate is not specified
    by vertical coordinate parameters but by providing a 3D field, that
    specifies the geometric height of every model grid point in meters. For more
    details, see Appendix in: https://www.dwd.de/DWD/forschung/nwv/fepub/icon_database_main.pdf

 3. Addition of new soil levels: New soil levels from ICON are added to
    METGRID.TBL.ARW for metgrid.exe. Details about the soil levels in ICON can
    be found in the documentation (see above). Compared to PR #154 all available
    model levels (soil moisture and temperature) are included now.

 4. Conversion of soil moisture units to those expected by the WRF model:
    The preprocessing in rrpr.F has to be modified to convert the units of
    column-integrated soil moisture provided by ICON from kg m-2 to m3 m-2 for
    WRF input.

 5. Addition of an identifier for DWD models (ICON) in rd_grib2.F and rd_grib1.F
    as done by @WQadam

This merge has been tested with ICON-EU (~6.6 km resolution) data, but it should
be able to run with ICON-D2 (~2.2 km resolution) and ICON-Global. For the latter
it needs to be converted from an icosahedral grid to a regular lat-lon grid.