This commit introduces a set of callback functions, packaged in an
abstract base class SatfuncCheckPointInterface<Scalar>, for querying
and populating the saturation function end-points that get probed by
the individual consistency checks. Member function
SatfuncCheckPointInterface::pointID(cellIdx)
translates the active cell index 'cellIdx' into a point ID, assumed
to be unique on at least the current MPI rank. This function will
return 'nullopt' if the 'cellIdx' is not eligible for this
particular end-point. This typically happens for the region based
tabulated (unscaled) saturation function checks when the 'cellIdx'
happens to be in a region that's already been visited. Member
function
SatfuncCheckPointInterface::populateCheckPoint(cellIdx, endPoints)
fills in (assigns) all data members of the 'endPoints' structure
with the pertinent values for the active cell 'cellIdx'.
We implement this interface for the tabulated/unscaled end-points in
derived class UnscaledSatfuncCheckPoint<Scalar> and for the scaled
end-points in derived class ScaledSatfuncCheckPoint<Scalar>. The
former keeps track of which saturation regions have been visited
and short-circuits its 'pointID()' member function based on that
information while the latter uses an instance of the former in order
initialise the 'endPoints' structure in its populateCheckPoint()
member function.
