Add "unique" marker to "Intersect annotations" workflow

Currently, the "Intersect annotations" workflow works as follows:

• There are annotations sets A and B
• Assume that a user selected to leave annotations from A that overlap B
  Expected result: by default we should get only one annotation per location.
  Actual result: if an annotation from A overlaps several annotations from B, the result will have several copies of the A annotation. The number of copies is equal to the number of annotations from B.

It should be possible to leave only one instance of annotation from A. Add an option to the workflow element and the corresponding workflow sample. Use this option by default.