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 


I understand why you made the change here, but I would prefer to keep the `into`, which is the default behaviour. You can add a note in the next line or in a `Notes` section that if `in_place=False`, the resulting node has a new ID, rather than `dst`.