Implement semiinteger & semicontinuous variable encodings

[pedromxavier]

Implement encoding for $x \in \{0\} \cup X$.

One option is to write

$$ \xi[\{0\} \cup X](\mathbf{y}; z) = z ~ \xi[X] (\mathbf{y}) $$

where $(\mathbf{y}; z) \in \mathbb{B}^{n + 1}$.