blockMesh: Change default location of blockMeshDict from constant/polyMesh to system

For multi-region cases the default location of blockMeshDict is now system/<region name>

If the blockMeshDict is not found in system then the constant directory
is also checked providing backward-compatibility

tutorials/multiphase/potentialFreeSurfaceFoam/oscillatingBox/system/blockMeshDict

@@ -0,0 +1,82 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+| =========                 |                                                 |
+| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
+|  \\    /   O peration     | Version:  dev                                   |
+|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
+|    \\/     M anipulation  |                                                 |
+\*---------------------------------------------------------------------------*/
+FoamFile
+{
+    version     2.0;
+    format      ascii;
+    class       dictionary;
+    object      blockMeshDict;
+}
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+convertToMeters 1;
+
+vertices
+(
+    ( 0 0 0)
+    (10 0 0)
+    (10 1 0)
+    ( 0 1 0)
+    ( 0 0 0.1)
+    (10 0 0.1)
+    (10 1 0.1)
+    ( 0 1 0.1)
+);
+
+blocks
+(
+    hex (0 1 2 3 4 5 6 7) (200 20 1) simpleGrading (10 0.1 1)
+);
+
+edges
+(
+);
+
+boundary
+(
+    freeSurface
+    {
+        type wall;
+        faces
+        (
+            (3 7 6 2)
+        );
+    }
+    walls
+    {
+        type wall;
+        faces
+        (
+            (0 4 7 3)
+            (2 6 5 1)
+            (1 5 4 0)
+        );
+    }
+    floatingObject
+    {
+        type wall;
+        faces
+        (
+        );
+    }
+    frontAndBack
+    {
+        type empty;
+        faces
+        (
+            (0 3 2 1)
+            (4 5 6 7)
+        );
+    }
+);
+
+mergePatchPairs
+(
+);
+
+// ************************************************************************* //