[BUG]: Restrict search on a support in `enumpoly` to be on the interior of the probability space

[tturocy]

The Newton's method implementation in enumpoly_solve generally behaves poorly in the case where an equation requires the solution to be on the boundary (i.e. the probability of one or more strategies or actions is zero).
This results in the branch-and-bound approach trying ever smaller search areas approaching the boundary.

Because any solution on the boundary at a given support is an "interior" solution on a strictly smaller support, we should
restrict the search space to be strictly on the interior of the space of probabilities, which will eliminate this behaviour.