Added in_place option to merge_nodes

This is in relation to the discussion that happaned in #1100. After this is merged, I will make the appropriate changes in #1100 


Hmm, sorry about missing this before, but at least a short docstring is required here: "add node `n` while updating the maximum node id". Also, "See networkx.Graph.add_node". Same comment for the other two inherited functions below.