GroupTree plugin: GRUPTREE/BRANPROP and terminal node config settings

[lindjoha]

Implement the option to choose whether to display the GRUPTREE or BRANPROP tree if both exists. One problem is that if a BRANPROP tree exists then the GRUPTREE nodes will not have group node pressures (GPR summary vectors).

[lindjoha]

and we could also add the possibility of specifying the terminal node (could be FIELD by default). FIELD might not always be exported (for BRANPROP) and it would also be possible to show only a part of the tree.