Add parameter to set numerical precision

[WillEngler]

Right now, Rootstock cavalierly decides on what precision to set. (Usually 32 point.) This is an important parameter to let users decide on. We need to expose a parameter for this in the RootstockCalculator interface and thread it through the deployed model configurations.

[OwenPriceSkelly]

I think this is tentatively closed by #214 -- latest refresh of env files now thread **kwargs through every setup function, so users can pass e.g. setup_kwargs={'default_dtype':'float64', 'mace_specific_kwarg':mace_specific_value} and they get passed to the mace_mp constructor without rootstock needing to enumerate all the possible options for all models (e.g. if mace calls it default_dtype and another calls it default_precision or something). We still set the sane/cavalier default but it's now always possible to pop open the hood if users want to fiddle with model options.