Set the current tree, if there are more than one sets of databases
$tree: It can be the default tree 'Top'.
Or 'Top_1' if you use -Top_1 option to load in other design
Or Top_ref in when using read_design("-ref", reference_netlist)
If $tree is not defined, the current database name is returned
Note:Implementation tree 'Top' has aliases of 'imp', 'IMP'
Reference tree 'Top_ref' has aliases of 'ref', 'REF'
set_tree("IMP"); # Same as the above
set_tree("Top_ref"); # Set to reference tree
set_tree("ref"); # Same as the above, set to reference tree
set_tree(); # Return the current database name. E.G. 'Top_ref'