Make member variables of Boundary.tmpl.h public

The members of other generated classes are public, so that they can be adapted during the simulation.

lbmpy_walberla/templates/Boundary.tmpl.h

@@ -178,7 +178,7 @@ private:
     void run( IBlock * block, IndexVectors::Type type{% if target == 'gpu'%}, cudaStream_t stream = 0 {%endif%});
 
     BlockDataID indexVectorID;
-
+public:
     {{kernel|generate_members(('indexVector', 'indexVectorSize'))|indent(4)}}
 };