generalize stack to more than 1 dimension

[gdementen]

Here are a few syntax experiments (but see also #30):

# 2D
stack([(('BE', 'M'), 1.0), (('BE', 'F'), 0.0),
       (('FO', 'M'), 1.0), (('FO', 'F'), 0.0)], ('nat', 'sex'))

# 3D
# a) flat list, label tuple
stack([(('BE', 1, 'M'), 1.0), (('BE', 1, 'F'), 0.0),
       (('BE', 2, 'M'), 1.0), (('BE', 2, 'F'), 0.0),
       (('BE', 3, 'M'), 1.0), (('BE', 3, 'F'), 0.0),
       (('FO', 1, 'M'), 1.0), (('FO', 1, 'F'), 0.0),
       (('FO', 2, 'M'), 1.0), (('FO', 2, 'F'), 0.0),
       (('FO', 3, 'M'), 1.0), (('FO', 3, 'F'), 0.0)],
      ('nat', 'type', 'sex'))

# b) recursive structure
stack([('BE', [(1, [('M', 1.0), ('F', 0.0)]),
               (2, [('M', 1.0), ('F', 0.0)]),
               (3, [('M', 1.0), ('F', 0.0)])]),
       ('FO', [(1, [('M', 1.0), ('F', 0.0)]),
               (2, [('M', 1.0), ('F', 0.0)]),
               (3, [('M', 1.0), ('F', 0.0)])])],
       ('nat', 'type', 'sex'))

[gdementen]

see also #455