Implement ZarrADMatrix objects

[Artur-man]

Hey @hpages, I am assuming you are ok with the naming of ZarrADMatrix, otherwise checks are passing except for some tests unrelated to this PR, e.g.

── Failure ('test-writeZarrArray.R:177:13'): writeZarrArray() ──────────────────
Expected `extract_array(A, index)` to be identical to `a3[1, 15:11, , drop = FALSE]`.
Differences:
12/15 mismatches (average diff: 12)
[1]   41 -  57 == -16
[2]   45 -  53 ==  -8
[4]   53 -  45 ==   8
[5]   57 -  41 ==  16
[6]  441 - 457 == -16
[7]  445 - 453 ==  -8
[9]  453 - 445 ==   8
[10] 457 - 441 ==  16
[11] 841 - 857 == -16

I have also corrected some surplus comments leftover from HDF5Array.

Should we add a few line of code to vignettes too? I did not add there anything for this PR, since we did not add any in the sparse matrix PR either but let me know.

[Bisaloo]

Proposed the same changes as in #10, does this make sense?

[hpages]

Thanks @Artur-man. Will check this soon...

Yeah adding a section in the vignette about the new ZarrSparseMatrix/ZarrADMatrix stuff is a good idea but let's do this in a separate PR.

Oh I forgot to mention that I'm using zarr_store for the first argument of ZarrSparseMatrixSeed() and ZarrSparseMatrix() to distinguish it from the first argument of ZarrArraySeed() and ZarrArray() which is zarr_path. Can you use zarr_store for ZarrADMatrixSeed() and ZarrADMatrix() too? Thanks.

BTW I think I will change ZarrArraySeed() and ZarrArray() interface at some point to also use zarr_store / name for consistency with ZarrSparseMatrixSeed() and ZarrSparseMatrix(). This will allow the user to access a dense matrix stored within an AnnData-style Zarr store in a way that is consistent with how they access a sparse matrix.

[Artur-man]

BTW I think I will change ZarrArraySeed() and ZarrArray() interface at some point to also use zarr_store / name for consistency with ZarrSparseMatrixSeed() and ZarrSparseMatrix()

Ah please ... it is actually really hard to work around this non-group based interface to zarr, we can at least have em in ZarrArray.

Will change to zarr_store now.

[Artur-man]

Proposed the same changes as in #10, does this make sense?

implementing now ...