* Simplified operation-relation deletion. Now we collect the relations
to delete into a vector, then iterate through that, thus solving issues
with iterator invalidation (+ aborts arising from that)
* DEG_foreach_ancestor_ID() was assuming that all dependencies were
OperationDepsNodes, when in fact, some could be TimeSource nodes
This commit adds a new method, DEG_foreach_ancestor_ID()
to accompany the existing DEG_foreach_descendent_ID().
It can be used to help print/collect all the ID's that
a given ID block depends on (i.e. all the datablocks that
need to be evaluated before the datablock of interest can
be evaluated)
