Adapt gammapyXray to gammapy 1.3

[lheckmann]

When running the example_nb.ipynb in gammapy-ogip-spectra, I needed to adapt two small things in gammapyXray to be compatible with gammapy version 1.3:

1. in models.py:
   L32ff:

            parameter = Parameter(
                name=par.name, value=par.val, frozen=par.frozen, is_norm=is_norm
            )

is_norm has to be removed since it has been remove from Parameter()

2. in ogip_spectrum_dataset.py
   L118ff:
   In the definition of npred_signal, all model_name occurrences should be updated to model_names to match the current format in SpectrumDatasetOnOff.npred_signal() in gammapy 1.3