Make passing a refSystem via the configuration optional

[t-ober]

Currently we need to pass a refSystem explicitly via the configuration:

  gridConfig {
    refSystems = [
      {
        sNom = "60 MVA"
        vNom = "10 kV"
        voltLvls = [
          {
            id = "MS"
            vNom = "10 kV"
          }
        ]
      }

I don't see why we can't make that optional and use sensible standard values so the user do not have to care about that if they don't want to.