Class MoleculeChooser

Known Subclasses:

AugmentedMoleculeChooser

presents user w/ a list of molecules currently loaded;
mode can be 'single', 'browse', 'multiple' or 'extended'.
Molecules can be selected from the list or by picking in the camera.

OK button returns a list of entries which have been selected.
Cancel returns an empty list. 
Typical usage is:
    ans = MoleculeChooser(self.vf).go()
    if ans !=[]:
        then get the value(s)
NB: this class doesn't grab the focus and binds picking w/
B1 to selecting the molecule in the MoleculeChooser.