Switching to the advanced configuration method and fixing spack build.

Several changes including switching to the new advanced configuration and improving spack building of icon.

conf/common/build.yml

+spack:
+  init: ""                                                # command to load spack environment, e.g. module load spack,
+                                                          # use spack/setup-env.sh if empty
+  url: https://github.com/spack/spack.git                 # url to download spack if necessary
+  branch: develop                                         # if downloaded, branch name to use
+  externals: ""                                           # list of packages we try to find with spack external find
+                                                          # In LMU we need to include slurm to this list.
+  compiler: "gcc@12.2.0"                                  # desired compiler for spack
+  root: "%HPCROOTDIR%/spack"                              # path to a spack install, will be downloaded to if not present
+  user_cache_path: "%HPCROOTDIR%/spack_user_cache_path"   # spack puts data here when bootstrapping, leave empty to use home folder
+  user_config_path: "%HPCROOTDIR%/spack_user_config_path" # spack puts data here when bootstrapping, leave empty to use home folder
+  disable_local_config: false                             # if true, spack installs into spack source dir
+
+icon:
+  # The command that will be used to build icon-nwp with spack
+  build_cmd: "icon-nwp@%ICON.VERSION%% %SPACK.COMPILER%"
+  # In LRZ we used the following command to point to a specific versio of openmpi:
+  # build_cmd: "icon-nwp@%ICON.VERSION%% %SPACK.COMPILER% ^openmpi/amct7nx"
+  # In LMU we used the following command to build the appropriate a version of openmpi works with slurm:
+  # build_cmd: "icon-nwp@%ICON.VERSION%% %SPACK.COMPILER% ^openmpi+pmi+legacylaunchers schedulers=slurm fabrics=ucx ucx+dc+dm+ib_hw_tm+mlx5_dv+rc+rdmacm+thread_multiple+ud+verbs"
+  version: master # The latest release at the moment of creating this file was 2.6.5-nwp0
+
+python_environment:
+  # Name of the virtual environment in the remote platform experiment folder
+  folder_name: python_environment
+  python_version: "3.8:"   # In spack notation use x: to indicate at least x
+  requirements:
+    # Because there's an issue with numba, for now we need to keep a specific version of numpy
+    - numpy==1.23
+    # Using enstools-compression to compress the outputs.
+    - enstools-compression
+    # We are using the f90nml library to manipulate name lists.
+    - f90nml