Handle non-csr/csc sparse matrices in anndata.write

[scottgigante-immunai]

Attempting to save an AnnData object with non-CSR/CSC sparse matrices throws an error:

File "/usr/src/singlecellopenproblems/openproblems/api/utils.py", line 58, in write_h5ad
      adata.write_h5ad(filename)
    File "/usr/local/lib/python3.8/site-packages/anndata/_core/anndata.py", line 1918, in write_h5ad
      _write_h5ad(
    File "/usr/local/lib/python3.8/site-packages/anndata/_io/h5ad.py", line 85, in write_h5ad
      write_elem(f, "X", adata.X, dataset_kwargs=dataset_kwargs)
    File "/usr/local/lib/python3.8/site-packages/anndata/_io/utils.py", line 220, in func_wrapper
      raise type(e)(
  TypeError: No method has been defined for writing <class 'scipy.sparse._coo.coo_matrix'> elements to <class 'h5py._hl.group.Group'>
  
  Above error raised while writing key 'X' of <class 'h5py._hl.group.Group'> to /

This PR fixes that by registering an appropriate method which converts all sparse matrices without an explicitly defined conversion to CSR.

[codecov]

Codecov Report

Merging #847 (c446a23) into master (bc9f886) will increase coverage by 0.04%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #847      +/-   ##
==========================================
+ Coverage   83.74%   83.79%   +0.04%     
==========================================
  Files          32       32              
  Lines        4399     4412      +13     
==========================================
+ Hits         3684     3697      +13     
  Misses        715      715              

Impacted Files	Coverage Δ
anndata/_io/specs/methods.py	84.98% <100.00%> (+0.51%)	⬆️

[ivirshup]

Hey. I think I'd be more inclined to say "that anndata is invalid and should have errored when you created it". I think many anndata operations would error with any of the sparse matrix types that don't do indexing right.

I would rather add broad support for them as they gain functionality, rather than later changing conversion behavior.

An alternative would be to do the conversion when the AnnData is constructed, but I think it's largely fine to leave this to the user.

[scottgigante-immunai]

I'd be fine with either throwing an error or automatically converting inside AnnData.X.setter -- just doing it in construction may not work as it's not that uncommon to set adata.X = func(adata.X).