Generic Reference Return Types

[mtao]

Our atomic operations have prototypes like

• EdgeMesh,Tuple -> EdgeMeshExecutorData
• TriMesh ,Tuple-> TriMeshExecutorData
• TetMesh ,Tuple-> TetMeshExecutorData

These need to be stored during the entire multimesh traversal so that compound operations can extract information from the last operation on any particular mesh. Because this involves different input and output types it's better to use std::variant - but the input types are all references so some manipulation around std::reference_wrapper is needed. This PR provides a somewhat ugly interface that will be kept internal for executing on generic variants.

A key function here is the implied ability to call as_variant<Traits>(Mesh&) that returns a std::variant<EdgeMesh&,TriMesh&,TetMesh&> so we can just call

// assume as_mesh_variant aliases to as_variant<MeshVarTraits>
Mesh m = ...;
auto mesh_ref_var = as_mesh_variant(mesh);

std::visit([&]template <typename T>(const T& mesh) {
using MeshType = unwrap_ref_t<T>;
if constexpr(std::is_same_v<MeshType, TriMesh>) {
// do tri mesh specific things
}
}, mesh_var);

If we use Functor classes (as seen in unit tests) then users can create custom multimesh visitors using functor structs + polymorphism and even cache the return types.

[codecov]

Codecov Report

Merging #438 (352c905) into main (cadf64c) will increase coverage by 0.76%.
Report is 64 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #438      +/-   ##
==========================================
+ Coverage   81.95%   82.71%   +0.76%     
==========================================
  Files         154      160       +6     
  Lines        4162     4496     +334     
==========================================
+ Hits         3411     3719     +308     
- Misses        751      777      +26     

Flag	Coverage Δ
wildmeshing	82.71% <100.00%> (+0.76%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...programming/ReferenceWrappedFunctorReturnCache.hpp	100.00% <100.00%> (ø)
src/wmtk/utils/metaprogramming/as_variant.hpp	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes