Axis[igroup_on_incompatible_axis] has wrong behavior

[gdementen]

added a warning for this for LGroups in #1154. I only handled the LGroup case because the code path is very different for IGroups and the 0.35 release is long overdue.

alt2_a = Axis('a=a1,a0,a2')
res = a[alt2_a.i[0:3]]
assert isinstance(res, LGroup)
# we SHOULD have this result
# assert res.key == ['a1', 'a0', 'a2']
# but we have this instead
assert res.key == slice('a1', 'a2')
assert res.axis is a