Add helper method to convert from SelectionField to AttributeType.

vtkSelectionNode::SelectionField is not same as
vtkDataObject::AttributeType though they have similar intents. Hence
adding methods to convert from one to another. At some point we should
fix vtkSelectionNode::SelectionField to match
vtkDataObject::AttributeType.

Change-Id: I2b21c646dc7252eae3a62699516dd79b4a590a2e
Merge topic 'fix_find_data' into master

e3229b0a Add helper method to convert from SelectionField to AttributeType.