GIPSL: Remove implicit variable multiplication in the mapping context

[maxkratz]

As of now, GIPSL implicitly multiplies the mapping's binary ILP variable to the terms given in, for example, a constraint with the mapping context. Example:

mapping m with someGtRule;

constraint -> mapping::m {
    <some constraint>
}

The problem arises that because of this, no other mappings can be accessed in a constraint with a mapping context because this would result in a quadratic equation. Hence, the following example is currently not possible:

mapping n with someOtherGtRule;
constraint -> mapping::m {
    <some constraint on self> <=> mappings.n->[...]
}

The current workaround is to use the pattern/rule of the mapping as context and overlap it completely within the filter expression as such:

constraint -> pattern::someGtRule {
    mappings.m->filter(match | match.nodes().x == self.nodes().x & [...])[...]
}

This results in a very "wordy" specification because a large amount of filter specification code is just used to get the one mapping that corresponds to the match of the pattern context.
As a result, this issue proposes to remove the implicit variable multiplication for the mapping context.

I will open another issue for another solution to the problem of "wordiness", but the implicit multiplication should nevertheless be remove IMO.