Initialize the collection for the multiple element selection node.

Conflicts:
	src/DynamoRevit/DynamoRevit.cs
Remove RevitData type and replace with DocumentManager.

Conflicts:
	src/DSRevitNodesTests/Interactivity/SelectionTests.cs
	src/DynamoTestFramework/DynamoTestFramework.cs
Remove dynRevitSettings.Revit.

Conflicts:
	src/DynamoRevit/DynamoRevit.cs
Remove dynRevitSettings.Doc and MainTransaction.
Remove unused RevitPersistenceManager project.

Conflicts:
	src/RevitPersistenceManager/Properties/AssemblyInfo.cs
Add DSDropDownSelection.