Add decision_variable_dependency to represent polynomial dependency between decision variables

[termoshtt]

Ready for binary encoding of integer variables.

• When an integer variable $x \in [0, 3]$ is binary encoded as $x = b_1 + 2 b_2$ with $b_1, b_2 \in {0, 1}$, $x$ will disappear from objective function and constraints.
• Thus, $x$ is not passed to solver which only solve with $b_1$ and $b_2$. So we have to reconstruct $x$ from $b_1$ and $b_2$, but how?
• decision_variable_dependency field in ommx.v1.Instance stores this relation, i.e. $x = b_1 + 2 b_2$ as a set of the ID of $x$ and a polynomial $b_1 + 2 b_2$.

[termoshtt]

More comprehensive test for evaluate dependency will be done when implementing log-encoding.