Add conservative flux divergence operator (FV::ConservativeFluxDiv)

[pressatojump]

Summary

This PR adds a conservative flux divergence operator for finite-volume discretizations in BOUT++. The operator computes the divergence of face-centered fluxes stored in FaceField3D objects.

Motivation

The conservative flux divergence operator is essential for finite-volume methods that ensure exact conservation properties. This is particularly important for:

• Transport equations where conservation is critical
• Shock-capturing schemes
• Coupled multi-physics simulations requiring consistent fluxes

Implementation Details

• New functions: FV::ConservativeFluxDiv with two overloads:
  • Basic version with boolean flag for boundary flux from field
  • Extended version with explicit boundary flux pointers for each dimension
• Finite-volume discretization: Properly accounts for metric factors (J, dx, dy, dz)
• Boundary handling: Complete support for X, Y, and Z boundary fluxes
• Parallel support: Includes ghost cell communication via mesh_facefield_comm

Changes

• Added include/bout/conservative_flux_div.hxx - header with function declarations
• Added src/mesh/conservative_flux_div.cxx - implementation
• Fixed FaceField3D virtual method overrides for C++17 compatibility
• Added unit tests (compile test + framework for functional tests)
• Updated build system files

Testing

• Compile test verifies the operator builds and links correctly
• Functional tests are implemented but disabled due to FakeMeshFixture size constraints
• All existing tests continue to pass

API Addition

namespace FV {
  // Basic version - uses field values at boundaries if bndry_flux=true
  Field3D ConservativeFluxDiv(const FaceField3D& F, bool bndry_flux = false);

  // Extended version - explicit boundary fluxes
  Field3D ConservativeFluxDiv(const FaceField3D& F,
                             const Field3D* bndry_flux_x,
                             const Field3D* bndry_flux_y,
                             const Field3D* bndry_flux_z);
}

[ZedThree]

This looks like it's built off next but going into master -- you probably just need to switch the base branch of the PR :)

[pressatojump]

Done @ZedThree :)