ENH: fused: 'fused' variants of explicit discretisation

Summary

WIP. Explicit 'fv' operations avoiding intermediate surface fields. This helps with

• memory allocations, caching
• rewriting loops to be cell-based

Details of new models (If applicable)

Drop-in replacement for Gauss (fusedGauss) of fvc::laplacian, fvc::grad, fvc::div, fvc::snGrad. Currently in separate library.

Risks

• slight truncation error difference since order of operations is different. laplacian only with scalar or vector weights supported so no tensor (for e.g. anisotropic heat-transfer)
• disallows potential support for boundary conditions on surface fields

src/fused/finiteVolume/Make/files

+fusedGaussLaplacianSchemes.C
+fusedLeastSquaresGrads.C
+fusedGaussDivSchemes.C
+fusedGaussConvectionSchemes.C
+fusedGaussGrads.C
+
+LIB = $(FOAM_LIBBIN)/libfusedFiniteVolume