Subdiagram improvements

[danoneata]

This PR addresses #113, but it's still work in progress. I've started by fixing the example from that issue and by following the plan laid out by Sasha. The next steps are to add the extra functionality.

TODO

• [x] get_location to retrieve the local origin of the Subdiagram
• [x] qualify diagram names by prefixing them with a given name
• [x] with_name (unclear whether this function can be used easily in Python, given that its most natural style involves nested lambda expressions)
• [x] boundary_from to return points on the boundary of a given Subdiagram given a direction vector
• [x] show_labels to show label names on top the diagram
• [x] Documentation

Deferred sub-tasks:

• style as an attribute to the Subdiagram class (since it was unclear to me whether this is useful for the current API)
• Draw labels on arrows (since it would be better served by a different PR)

[srush]

I agree withName is a bit too aggressive for python. It looks like there is also a with_names function though that might be better.

[danoneata]

It looks like there is also a with_names function though that might be better.

Yes, you are right, with_names looks more appropriate for us. I've replaced with_name with with_names and added some examples (translated from Haskell) in the documentation.

I think I'll move the task of adding labels to arrows to a separate issue (and PR) since it appears a bit more involved at first sight, especially if we want it to work on arrows with arbitrary trails (shafts).