The nearWallDist MeshObject is now deleted on mesh-change rather than updated
which is more efficient for cases with multiple mesh changes, e.g. motion,
stitching and mapping by avoiding unnecessary updates.
As a consequence of this change the wallDist::y() volScalarField reference
should not be cached across mesh changes so the turbulence models now obtain the
y field as required from the new momentumTransportModel::y() function, the
original near-wall distance function is now named momentumTransportModel::yb()
to clarify that it is the wall distance of the boundary cells.
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