local interpeter assumes comparator operators return bool

[danbgoldman]

The current implementation of evaluate_condition only works properly if the result of a comparator operation is a boolean.

Some modules may return other types for comparisons, especially if they are building their own symbolic representation of an expression. For example, the pulp library for mixed linear integer programming returns an LpConstraint object from a comparison. This causes an exception here, because there is no bitwise and operator for bool & LpConstraint.