Add option to set Indiv Start Nodes for the icicle model

[sslupsky]

Is your feature request related to a problem? Please describe.
The icicle model allows specifying multiple strings and the nodes per string but does not allow the user to specify the Indiv Start Nodes. It would be helpful to be able to do so.

[cybercop23]

None of the native models do. There was an effort to start that for circle and star with matrix being next. #4554