GIPSL: Shortcut for filter overlap

[maxkratz]

Currently, there are many constraints in GIPSL that have filter expressions for overlapping the GT pattern/rule (context) with the set of mappings (that also have the same GT pattern/rule) as context. This is in most but not all occurrences because of #195.

Besides the "complete" overlap of a pattern/rule context to a mapping, there is sometimes the need to overlap only parts of the two. Example:

pattern pa {
    o1 : Object
    o2 : Object
    x : SomeOtherType
}

pattern pb {
    o1 : Object
    o2 : Object
    y : AnotherType
}

mapping m with pa;

constraint -> pattern::pb {
    mappings.m->filter(map | map.nodes().o1 == self.o1 & map.nodes().o2 == self.o2)->[...]
}

These filter expressions only get longer and more "wordy" if the respective patterns/rules have more nodes that must be overlapped.
It would be nice to have some kind of shortcut for the overlap, e.g.:

constraint -> pattern::pb {
    mappings.m->overlap(o1=o1 & o2=o2)->[...]
}