Inplace subset of anndata object doesn't update AxisArrays

[fhausmann]

Inplace subsetting of an anndata object does not update obsm or varm. They updated anndata objects gets new AxisArrays objects (with the correct shape), but the old objects don't get updated and even if they are still referencing the updated anndata object.

from anndata.tests.helpers import gen_adata

a = gen_adata((200, 100))
o1 = a.obsm
a._inplace_subset_obs(slice(50))
o2 = a.obsm

assert o1.parent is o2.parent
assert o1["array"].shape == o2["array"].shape
id(o1) == id(o2) # False

Originally posted by @ivirshup in #363

[fhausmann]

As @ivirshup was mentioning here (#363 (comment)) possible fixes include:

it would either make sense that it change, or it no longer referred to the parent of the wrong shape.