ENH: use thisDb reference when referencing/creating finite-area fields

COMP: remove faMesh::operator()() in favour of mesh() or thisDb() instead

- makes the purpose and usage clearer

src/finiteArea/interpolation/edgeInterpolation/schemes/NVDscheme/faNVDscheme.H

@@ -119,7 +119,7 @@ public:
             NVDweight(is),
             edgeFlux_
             (
-                mesh().objectRegistry::lookupObject<edgeScalarField>
+                mesh.thisDb().lookupObject<edgeScalarField>
                 (
                     word(is)
                 )

src/finiteArea/interpolation/edgeInterpolation/schemes/blended/blendedEdgeInterpolation.H

@@ -106,7 +106,7 @@ public:
             upwindEdgeInterpolation<Type>
             (
                 mesh,
-                mesh().objectRegistry::lookupObject<edgeScalarField>
+                mesh.thisDb().lookupObject<edgeScalarField>
                 (
                     word(is)
                 )

src/finiteArea/interpolation/edgeInterpolation/schemes/upwind/upwindEdgeInterpolation.H

@@ -97,7 +97,7 @@ public:
             edgeInterpolationScheme<Type>(mesh),
             faceFlux_
             (
-                mesh().objectRegistry::lookupObject<edgeScalarField>
+                mesh.thisDb().lookupObject<edgeScalarField>
                 (
                     word(is)
                 )