Remove cells with less than 3 models in the "Ensemble details" table

[Bisaloo]

At the bottom in the ensemble report:

In this example, hospitalisations in Italy should not be shown with 1 since models with less than 3 models are not included in the ensemble (since https://github.com/covid19-forecast-hub-europe/covid19-forecast-hub-europe/pull/2040).

I see two ways to do this:

• add a column in the criteria/ tables to indicate that models concerning targets with less than 3 models are excluded from the ensemble.
• manually remove these models from the "Ensemble details" table in https://github.com/covid19-forecast-hub-europe/covid19-forecast-hub-europe/blob/main/code/reports/rmdchunks/nmodels-ensemble.Rmd. In we go this route, we should probably implement #2041 first.

[kathsherratt]

Completed by @sbfnk in EuroForecastHub and in this repo here