Use "xcube level" tool with S3

[forman]

This PR closes #617 and addresses #516:

• The xcube level CLI tool has been rewritten from scratch to make use
  of xcube filesystem data stores. (Allow creating pyramids on S3 #617)

• Deprecated numerous classes and functions around multi-level datasets.
  The non-deprecated functions and classes of xcube.core.mldataset should
  be used instead along with the xcube filesystem data stores for
  multi-level dataset i/o. (Unify xcube dataset i/o #516)

  • Deprecated all functions of the xcube.core.level module
    • compute_levels()
    • read_levels()
    • write_levels()
  • Deprecated numerous classes and functions of the xcube.core.mldataset
    module
    • FileStorageMultiLevelDataset
    • ObjectStorageMultiLevelDataset
    • open_ml_dataset()
    • open_ml_dataset_from_object_storage()
    • open_ml_dataset_from_local_fs()
    • write_levels()

• Support for multi-level datasets has been improved:

  • Introduced new parameters for writing multi-level datasets with the "file", "s3", and "memory" data stores. They are
    • ...
    • num_levels: If given, restricts the number of resolution levels
      to the given value. Must be a positive integer to be effective.

Closes #617.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in CHANGES.md
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

FYI @AliceBalfanz @pont-us

[codecov-commenter]

Codecov Report

Merging #649 (7923c82) into master (a19aa3c) will decrease coverage by 0.00%.
The diff coverage is 95.04%.

@@            Coverage Diff             @@
##           master     #649      +/-   ##
==========================================
- Coverage   91.96%   91.95%   -0.01%     
==========================================
  Files         298      298              
  Lines       27335    27400      +65     
==========================================
+ Hits        25138    25197      +59     
- Misses       2197     2203       +6     

Impacted Files	Coverage Δ
xcube/cli/level.py	91.83% <86.20%> (-8.17%)	⬇️
xcube/core/mldataset.py	79.32% <94.73%> (+1.03%)	⬆️
test/cli/helpers.py	100.00% <100.00%> (ø)
test/cli/test_level.py	100.00% <100.00%> (ø)
test/core/test_mldataset.py	99.39% <100.00%> (+0.03%)	⬆️
xcube/core/level.py	84.04% <100.00%> (-0.57%)	⬇️
xcube/core/store/__init__.py	100.00% <100.00%> (ø)
xcube/core/store/fs/impl/mldataset.py	89.74% <100.00%> (+0.77%)	⬆️

---

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[TonioF]

Minor suggestions for improvement, but overall it looks good!

[TonioF]

The reason does not match.

[forman]

It does, becasue it was used when writing only.

[TonioF]

As there is the theoretical possibility that we might have different deprecation versions in the same module, I'd propose to consider this in the name. Or even put this in a dedicated module somewhere, so we really only have to declare it once.

[forman]

This is a private constant and it stands for exactly one constant value used here. If we need more versions one day, we can easily add more constants.