This module permits to rapidly  change the tree in a 2D-wavelet packet
transform. 

See the documentation on \verb+wp2dmktree+ for details on the coding of
quad-tree in MegaWave2, and the documentation on \verb+wp2ddecomp+
for details on the implementation of the 2D-wavelet packet transforms.
One can also check if a \verb+Cimage+ actually
describes a quad-tree with the module \verb+wp2dchecktree+.

Notice that NO assumption is made on the target tree. It just needs to
describe a valid quad-tree.
Also, the same \verb+Wpack2d+ can be used for the input and the output.