cylc gui graph view: crop purely-base-node-points

This closes #1155.

In `cylc gui`'s graph view, base nodes are automatically cropped if all the 
nodes in their cycle point are base nodes.

This helps with #1155, although you may still get base nodes in the first one 
or two cycle points sticking around because their cycle points contain immortal 
"succeeded" tasks.

@hjoliver, please test (to see if the behaviour is what we want) and review.
I think you mean "base nodes" here, not "proxy nodes"?