Better Dot rendering of closures

[acl-cqc]

closes #101 - the BRAT graph is fine, it was just the rendering that was terrible.

• Exclude nodes captured by a box from being in the box. This was how multiple Sources were ending up in the same box as a Target. Instead each Source should now end up in the same box as its Target.
• Add edge from Box node to Target. This is probably redundant now I have properly fixed the previous.
• Shorten rendering of labels: "Box sourcenodename targetnodename" is now just "Box", similarly Eval. The names are redundant as they are shown by the dotted edges. (So maybe the Box-Target edge does have a purpose, allowing label to be shortened?)
• Properly nest clusters, i.e. when a Box node B1 is in a cluster C1, the associated cluster C2 with B1's Source+Target should now be a subcluster of C2

This piccy demonstrates effect on function f in closures.brat, admittedly for the pic I also shortened PatternMatch but really we need to handle those properly - a TODO for a followup

Previously the middle (between left and right) Source had been put in the LHS cluster, causing the confusion, and of course the clusters were not nested.

[croyzor]

?!

[acl-cqc]

🤣 now defunct but it was temporarily to silence some error messages. Thanks for spotting :) now removed!