Add option to input a user defined array for branchextraction in maniupulate_branch

[aslakbergersen]

In manipulate branch there could be added an option to use a predefined array for branch extraction, instead of using vmtkbranchextractor.

[hkjeldsberg]

Just to clarify, this means the user can input an array describing any centerline (either within the current model OR an arbitrary centerline), and use this as input for branch manipulation (?)