Enh: postprocessing: coarse-graining, call path listing

[Technologicat]

Pyan3 could use some postprocessing support:

• In a mid-sized or large project, it's currently easy to lose sight of the forest for all the trees (pun not intended). The visualization of all individual functions provides way too much detail to get a useful overview of the whole codebase. It would be useful to restrict the view to show just the uses-relations between classes, or even just modules, hiding the detail.

• See https://github.com/Technologicat/pyan/issues/1 about listing all call paths. The comments include a very rough first cut of untested example code explaining how this could be done.

[Technologicat]

Added a module-level dependency visualizer modvis.py.

This was simpler to implement as a separate program that uses the same output machinery as the main pyan program. Should also run faster, since all it needs to look at are Import and ImportFrom AST nodes.