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Always verify that map entries exist before accessing the shared
pointer held by the map.

Some utility methods for creating SMTK entity records in common arrangemnts.

Track regions not connected by edges but by the sweepline.

Disconnected regions and loops are correctly detected.
Disjoint regions are not merged together; that requires
additional work.

Sessions should not have to make every operator a friend.
This adds methods to the base polygon Operator class to
provide access to the Session's storage.

Python already has a "CreateFace" function (which works for
CGM) but this commit adds "CreateFaces" for expediency in
testing the polygon session. The operators should be reconciled.

New Modeling Kernel
-------------------

This commit adds a new modeling kernel based on
[Boost.polygon](http://www.boost.org/doc/libs/1_58_0/libs/polygon/doc/index.htm).
It currently supports a very limited set of operations:

+ Creation a model.
+ Creation of model vertices.
+ Creation of model edges.

Internally, an edge split operation is provided but not yet exposed.

Miscellany
----------

+ Fix a bug in attribute association.
  Because model entity items in attributes may have a membership mask,
  calling `setValue()` does not guarantee that the association will
  occur. Return the proper boolean or infinite recursion will occur
  inside `Attribute::associateEntity()`.
+ Add a Python-callable variant of ImportJSON::ofLog.
+ Have `smtk.simple` hang on to operator results so that scripts can access them if they need to.
+ Fix error log printout in `smtk.simple` API.
+ Have python test `addToScene()` method return pipeline objects.