Refactor select.py to use object-oriented API

[frreiss]

The file graph_def_editor/select.py contains various routines for selecting subsets of a graph's nodes and/or tensors.

A few issues with the current organization of this file:

• Adds 18 functions to the top-level gde namespace
• Entry points are monolithic -- see, for example, select_s()
• Return values are not in a single consistent format
• No easy way to represent a selection expression as an object

To address the above issues and to facilitate the development of the "pattern" part of pattern-action rewrite rules, we should refactor these functions into a more object-oriented design:

• Add a base class, Selector, for selection expressions over a graph. The base class will have methods to:
  • Get information about what types of things (tensors, nodes, or both) the expression can select
  • Retrieve the current set of selected graph objects as a Python list
  • Retrieve the current set of selected objects as a SubgraphView
• Make the existing functions return subclasses of Selector
• Put existing functions under a new namespace gde.select
• Remove unnecessary subroutines from the top-level package
• Refactor existing test cases to use the new APIs