Create method which merges two networks

[jessLryan]

we need a method which takes two pypsa networks n1 and n2 as input and returns a third network n3 which is made from 'merging' n1 and n2 - most of the code for this is pushed to the Jess-WorkInProgress branch. This method should also return a mapping from each of the original buses (in n1 and n2 respectively) to the buses of n3, so we'll need to create a custom object to return both of these things (see for example the existing clustering methods in pypsa, they do something similar). Note that the existing code in Jess-WorkInProgress currently changes the names of buses in n1 and n2 - we actually want these name changes to be local within this method, and not permanent changes to n1 and n2.

[jessLryan]

Note that the objective here is to divide up the code into sensible components. We should do this quite urgently, and expect to need to do it several more times in the future as the best structure of the project becomes clearer.