Unnesting is incorrect in the case of x-x2

[Broes De Cat]

If both x and x2 are variables, they will certainly be ground when they atomgrounder reaches them. In isTwoValued check?

[Broes De Cat]

Fixes #265: Performance bugfix: unnest less.

Terms which are in fact two-valued should in most cases not be unnested during graphing (except during symbolc propagation ...)

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