Recover a version of FF16Plus

[dfalster]

We removed PlantPlus object (#159) as it was impeding generalisations about strategy.

Now that all that has settled, would be good to make a new version of FF16 that enables the type of probing we did with plant plus.

PlantPlus gave access to more variables in the growth model. This is now supported via ability to track auxiliary variables in the internals. Following these changes we need to:

• [ ] Extend the FF16 model to include the full list of auxiliary variables previously available
• [ ] Benchmark against quick version to check speed difference
• [ ] Update vignettes
• [ ] Update tests against reference comparison plant for the additional variables
• [ ] Compare a plant running with and without additional variables against each other (should be identical in core vital rates
• [ ] Reinstate tests disabled during development

[dfalster]

Key functionality introduced via 65da50ab8550436ac32d50817b871903f86d469b

@itowers1 recent analyses using plant have effectively demonstrated the capability.