This function aims at easily exploring very large trees. The main argument is
a phylogenetic tree, and the second one is a logical indicating whether a waiting message should be printed while the calculation is being processed.
The whole tree is plotted on the left-hand side in half of the device. The
subtree is plotted on the right-hand side in the other half. The user clicks on the nodes in the complete tree and the subtree corresponding to this node is ploted in the right-hand side. There is no limit for the number of clicks that can be done. On exit, the subtree on the right hand side is returned.
To use a subtree as the new tree in which to zoom, the user has to use the function many times. This can however be done in a single command line (see example 2).