mappedPatchBase: Add "samePatch" option

This option means that a one field can be mapped to another within the
same patch without specifying the patch name. E.g.:

    walls
    {
        type            mappedValue;

        //neighbourPatch  walls; // <-- Previously required. Still supported.

        samePatch       yes;     // <-- New alternative specification

        field           T.liquid;
        value           $internalField;
    }

This is useful when the boundary condition is specified using a regular
expression for the patch name.

    "wall_.*"
    {
        type            mappedValue;

        //neighbourPatch  ???;   // <-- No unique name can be given

        samePatch       yes;     // <-- Still works

        field           T.liquid;
        value           $internalField;
    }

tutorials/modules/multiphaseEuler/Grossetete/0/T.gas

@@ -35,7 +35,7 @@ boundaryField
     walls
     {
         type            mappedValue;
-        neighbourPatch  walls;
+        samePatch       yes;
         field           T.liquid;
         value           $internalField;
     }