Added GafferScene.PrimitiveVariables node.

This allows arbitrary primitive variables with constant interpolation to be added to objects.
Added a space into the PrimitiveVariables menu item.
Improved PrimitiveVariables node documentation.
Merge pull request #791 from johnhaddon/primitiveVariables

Added GafferScene.PrimitiveVariables node.
Added string matching function.

This is extracted from PathMatcher and will be used as the basis for unifying string matching across Gaffer as required by #707.
Added numericSuffix() function to StringAlgo.h.

This allows us to remove some duplicated code from GraphComponent and BranchCreator.
Added a Duplicate node to GafferScene.