Error when applying relational order to categorical parameters

[MLopez-Ibanez]

Using relational order operators to compare categorical values should not be allowed. For example, a parameter like:

a "" c (0, 5, 10, 20)

and a condition like a > 10 will return TRUE when a is 5, because a is actually "5" so the comparison is lexicographic.
We reading the condition, we should check if both a categorical parameter and a relation parameter appear in the condition, if so, we need some way to check that the relation operator is applied to the categorial parameter. We should not error for this:

a "" c (0, 5, 10, 20)
b "" i (0, 10)
c "" r (0,1) | a != 5 && b < 4

This applies also to expressions in forbidden.txt

(this was bug 2 in BUGS)