question/cppcheck: passing node by const value instead of const ref

[timosachsenberg]

In the _M_get_j_max / min functions, the node is passes by const value:
https://github.com/nvmd/libkdtree/blob/master/kdtree%2B%2B/kdtree.hpp#L814

Our run with cppcheck complained about it so I wanted to check back if this is intended.