Warn about unconstrained integer variables

[MaxOstrowski]

This addresses the issue with undefined variables that only occur in the body of rules and are not statically "defined".
@rkaminsk would it be enough to raise a warning somewhere in init if a domain of a variable has an upper/lower bound that is min/max (given by config) ?

[rkaminsk]

@rkaminsk would it be enough to raise a warning somewhere in init if a domain of a variable has an upper/lower bound that is min/max (given by config) ?

No this would not be enough because in encodings the bounds of variables can be set exactly to these values.