Support real types + numerical stability fix in NS (#254)

- Real types are supported by appropriate inicialization.
- A feature of the XTI spanning tree structure is removed to ensure
numerical stability (could cause problems using integer types).
The node potentials are updated always on the lower subtree,
in order to prevent overflow problems.
The former method isn't notably faster during to our tests.

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 2 	3 	6 	  10

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 2 	6 	8 	  8

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 3 	6 	10 	  25

 4 	7 	11 	  15

 5 	7 	12 	  15

 6 	7 	13 	  18

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