Add forecast hybrid

[seabbs]

• Added a model wrapper for the forecastHybrid package
• This package does ensembles of forecast models. Unfortunately, it does not produce samples and estimates CIs either using an unweighted mean or assuming the max/min of the candidate models. This makes the rather nice cross-validated ensembling functionality a bit pointless but matters less if using an equal weights ensemble which will be the baseline case.
• Suggest merge so we can use now and improve the handling of uncertainty in a later PR if we continue to use this package.
• Dopped several tests that fail based on the seed for now whilst a better solution is sort.