meshQualityDict: Switch off the minVol control by default

It is not clear for what cases the minVol control is useful or necessary and for
some cases it causes problems with snapping and layer addition if not set to a
sufficiently small value.

tutorials/mesh/foamyHexMesh/blob/system/meshQualityDict

@@ -38,7 +38,7 @@ minTetQuality       1e-30;
 //- Minimum cell pyramid volume relative to min bounding box length^3
 //  Set to a fraction of the smallest cell volume expected.
 //  Set to very negative number (e.g. -1e30) to disable.
-minVol 1e-10;
+minVol -1e30;
 
 //- Minimum face twist. Set to <-1 to disable. dot product of face normal
 //  and face centre triangles normal