Allow shape accessors on X et al.

[johnkerl]

• TileDB string-dimensioned sparse arrays don't have a .shape() -- it does not exist (see also .shape attribute of arrays created with from_h5ad generates error #10)
• People will want to know the 'shape' of X, obsp, varp -- I suspect it will be negative UX if people who are accustomed to getting anndata.X.shape cannot get soma.X.shape()
• Effectively the shape of soma.X is (soma.obs.shape()[0], soma.var.shape()[0]) and the shape of soma.raw.X is (soma.obs.shape()[0], soma.raw.var.shape()[0])
• What we can do is within the SOMA class:
  • Pass references to soma.obs and soma.var to the soma.X constructor
  • Have an X method called .shape() which will do what bullet Read API part 1: C++ API query and buffer management libraries #3 above says

Note: On PR #129 we have shape-by-scan which is O(n). For the TileDB core 2.10 timeframe we'll have an O(1) shape accessor.

[johnkerl]

Shape accessors are in place; optimization is a separate task.