Remove plot.OutputsModel from NAMESPACE

The plot.OutputsModel() function has been export again in order not to break the reverse dependency of the current version of airGRteaching (v0.2.9.25) available on the CRAN (see #89 (closed), ab853bd3). This break has already been fixed in the development version of airGRteaching (v0.2.10) that will be submitted to CRAN shortly after airGR v1.6.

Once the new versions of airGR and airGRteaching will be available on CRAN. This function can be removed from the NAMESPACE again from the development version of airGR.