MULES: Added optional boundaryExtremaCoeff

This provides more flexibility in specifying the allowed internal and boundary
extrema.

For driftFluxFoam and other settling problems it is beneficial to set the
boundaryExtremaCoeff to 1 to allow rapid accumulation of the partials on the
bottom wall (which was the previous default behaviour) but this is not suitable
for many Euler-Euler cases for which a uniform etrema coefficient is preferable,
either 0 or a small value.

Now by default boundaryExtremaCoeff is set to extremaCoeff which defaults to 0
which provides the behaviour before

OpenFOAM-dev commit cb2bc60fa5

and the driftFluxFoam tutorials have been updated adding

        boundaryExtremaCoeff 1;

to the MULES controls in fvSolution so reproduce the previous behaviour.

tutorials/multiphase/driftFluxFoam/RAS/dahl/system/fvSolution

@@ -26,6 +26,8 @@ solvers
         nLimiterIter    3;
         alphaApplyPrevCorr  yes;
 
+        boundaryExtremaCoeff 1;
+
         solver          smoothSolver;
         smoother        symGaussSeidel;
         tolerance       1e-6;

tutorials/multiphase/driftFluxFoam/RAS/mixerVessel2D/system/fvSolution

@@ -26,6 +26,8 @@ solvers
         nLimiterIter    3;
         alphaApplyPrevCorr  yes;
 
+        boundaryExtremaCoeff 1;
+
         solver          smoothSolver;
         smoother        symGaussSeidel;
         tolerance       1e-6;

tutorials/multiphase/driftFluxFoam/RAS/tank3D/system/fvSolution

@@ -26,6 +26,8 @@ solvers
         nLimiterIter    3;
         alphaApplyPrevCorr  yes;
 
+        boundaryExtremaCoeff 1;
+
         solver          smoothSolver;
         smoother        symGaussSeidel;
         tolerance       1e-6;