Merge pull request #1591 from Benglin/master

Fixed auto-merge issue
fixed migration for formula node
hide ironpythonevaluator backend method
cleanup some code in DSFunction
cleanup some code in DynamoController
Cleanup visualization manager pause APIs.

Make the IsUpdating flag setter internal and add two methods to the
interface Pause() and UnPause(). Use these new methods to defer
visualization updates during model loading. Also adds an check whether
visualization manager is paused to ignore node property updates.
Add two interfaces.

ICleanup interface to be used on objects requiring shutdown cleanup.
IVisualizationManager interface moved from its previous location in
VisualizationManager.
Remove IVisualizationManager from VisualizationManager.
Fix backwards pause and unpause on drag selection.
Initialize cachedMirrorData with a null.
Add the previously created interfaces to the project.
Don't create null MirrorData objects.

Standardize on return null for CachedValue.
Make the print value method check for null CachedValue.
added execution interval framework
Merge branch 'master' of github.com:DynamoDS/Dynamo
Removed Revit.GeometryObjects, renamed Revit.References to Revit.GeometryReferences so as to be more explicit, all GeometryObjects are converted to ProtoGeometry types
Remove commented out Revit Element selection methods
One GeometryObjectSelector method to rule them all, converts all GeometryObjects to ProtoGeometry geometry
Merge branch 'master' of https://github.com/DynamoDS/Dynamo
reverted resharper base qualifier fix
Fix the build