[WIP] Allow specifying element range in project_vector

[hugary1995]

Using std::optional to avoid breaking API.
This is needed in moose element subdomain modifier where we need to project a function onto a subset of the mesh.

[hugary1995]

I will add some tests later. @roystgnr please let me know if this looks okay to you before I go too far.

[roystgnr]

Sorry I missed the comment here!

Definitely we want to add this feature.

I'd have made the range argument a pointer with a nullptr default, but out of force of habit rather than because it's a good idea, and I'm happy with std::optional here. Someone correct me if I'm wrong, but best practices now is to use std::optional for any object that's lightweight enough to not worry about copying, right? (and probably also for heavyweight objects, eventually, when C++26 adds optional<T&> support?) Our ranges are currently like 48 bytes, so the copies should be fine.