Unit tests, first functions of the algorithm

I know that the second unit test is a big function, but it is really simple.

The sentence `John Smith lives in United Kingdom` is interesting for two reasons:

* It shows a collapsed tree, with the `prep_in` dependency (i.e. there is no node for the word `in`).
* We would like to perform NamedEntityTag merging in a near future (i.e. merge `John Smith` into one single node, same thing for `United Kingdom`).

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* Add a function to merge two nodes (e.g. two nodes with a same NamedEntityTag).