Feat: adding support for sharded precomputed

[xgui3783]

This is a PR adding support for sharded precomputed

This PR should (hopefully) support both read/write from local/http, much like the existing FileAccessor and HttpAccessor.

I will be polishing the PR by:

• [x] adding test coverage
• [x] update usage example

in the coming days.

I would like already to gauge if:

• neuroglancer-scripts is interested to have the ability to read sharded precomputed data source
• feedbacks on code

[codecov[bot]]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (a6bce3f) 92.61% compared to head (7f1d4b1) 93.96%.

Files	Patch %	Lines
src/neuroglancer_scripts/sharded_file_accessor.py	98.23%	3 Missing and 2 partials :warning:
src/neuroglancer_scripts/scripts/scale_stats.py	55.55%	3 Missing and 1 partial :warning:
src/neuroglancer_scripts/accessor.py	90.90%	1 Missing and 2 partials :warning:
src/neuroglancer_scripts/sharded_http_accessor.py	95.77%	2 Missing and 1 partial :warning:
src/neuroglancer_scripts/sharded_base.py	99.17%	1 Missing and 1 partial :warning:
src/neuroglancer_scripts/volume_reader.py	81.81%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #35 +/- ## ========================================== + Coverage 92.61% 93.96% +1.34% ========================================== Files 25 28 +3 Lines 1490 2138 +648 Branches 219 308 +89 ========================================== + Hits 1380 2009 +629 - Misses 63 75 +12 - Partials 47 54 +7 ```

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[xgui3783]

@ylep I believe the PR is mostly ready. Can you take a look when you get a chance?

[xgui3783]

@ylep I have added an additional commit, which hopefully address most if not all of your concerns.

can you take another look when you get a chance?

[xgui3783]

Thanks @ylep for the quick response. I would fully support a 1.2.0 release.