Docstrings
Contracts interface
Merge pull request #512 from ChrisBeaumont/contracts

Setup PyContracts interface
Better treatment of simple links after plots created.

Introduces a new message to communicate that a ComponentID is replaced,
which allows data viewers to intelligently update their displays
Added contracts
Subset contracts
More contracts
Allow series in component_like
Fixes to support updates to Ginga canvas and glue z-order
Merge pull request #515 from ejeschke/master

Fixes to support updates to Ginga canvas and glue z-order
test fixes
Merge pull request #508 from ChrisBeaumont/link_fix

1D linking does not work both ways
Merge pull request #516 from ChrisBeaumont/contract

More contract annotation