https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#con1-by-default-make-objects-immutable

change the datatype of the variable numberOfNodes aka numberOfLBnodes (fka size_Mat) to unsigned long long ... with respect to large grids (>100e^6 Nodes per Level) ... in this regard the 27 directions are changed to size_t and some control variables are also changed to size_t