Make the node schema pointer getter const

[JustinPrivitera]

https://github.com/LLNL/conduit/blob/fa0f8d3738b0c4cef95a0d05003f3b80aa381f23/src/libs/conduit/conduit_node.hpp#L3777

This should be const like the reference version of this function. The problem with doing this is that several areas of Conduit are built on this being non const. So far I have discovered tests (t_conduit_node_move_and_swap) and the python interface that depends on this being non const.

https://github.com/LLNL/conduit/blob/fa0f8d3738b0c4cef95a0d05003f3b80aa381f23/src/libs/conduit/python/conduit_python.cpp#L6092