#484 Undefined behaviour when no fabm.yaml/fabm.nml is present

[Carsten Lemmen]

The FABM component needs a configuration file either fabm.nml or fabm.yaml or any other name specified with fabm_nml=XXXX in fabm_pelagic.nml.

We observed that it does not, however, take fabm.yaml as default if fabm.nml is absent, leading to undefined behaviour at fabm initialization (i.e. segfault). This needs to be fixed and an error message needs to be provided.