Support for multiple types of `MeshBlockData`

[lroberts36]

There are a number of use cases for allowing for multiple types of variables (e.g. mixed precision solvers, flagging). During the 2022 Parthenon meeting, a discussion was had about the best path for implementing this and what the needs of various users were. The consensus was that this is straightforward to implement, but it is not currently high priority. Some thoughts:

• Currently, MeshBlockData<T> is templated on type T even though currently T = Real is the only thing that is ever used. When a new MeshBlockData<T> object is created, it adds a field for every entry inresolved_packages->AllFields(). When there are multiple types allowed, there should be some way of specifying the type of a field in its Metadata and this loop should check that the field has the type T before adding it to the MeshBlockData.
• The consensus path for allowing for multiple variable types was including multiple types of Mesh*Data<T> in a dataDataCollection for every stage and adding a method template<class T = Real> DataCollection::Get(const std::string& stage) that gets the Mesh*Data for type T at stage.
• Then we also need to template packs on variable type as well. Each pack would only contain one type of variable.
• Also need to template things like prolongation and restriction for different floating point types.

[brryan]

One small concern: If we require specifying types through Metadata then won't downstream developers be unable to enroll types that aren't already hardcoded into parthenon, for example a downstream code-specific enum? I think this wouldn't be a concern if we instead specifed type via a template parameter on the AddField function.

[lroberts36]

I was thinking that there might be some clever way to store a generic type in the metadata (not sure if this is possible? although my general belief is that all is possible in c++ at compile time), rather than doing this through metadata flags.