Refactor compressors to be in separate files

[asinghvi17]

This PR reformats the compressors to be in a folder with separate files, and documents the "compressor API".

(Previous commits added the fletcher32 filter as a compressor, but on review that should be a filter so I'll put it in a new PR.)

[coveralls]

Pull Request Test Coverage Report for Build 10492471554

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 51 of 55 (92.73%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.5%) to 86.192%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Compressors/blosc.jl	18	19	94.74%
src/Compressors/Compressors.jl	22	25	88.0%

Totals
Change from base Build 8981180163:	-2.5%
Covered Lines:	799
Relevant Lines:	927

---

💛 - Coveralls

[asinghvi17]

I've implemented fletcher32 compression in this last commit, and am testing it with unit tests copied from the Python package. I'm not sure what the best way to load it in integration testing is though, so could use a hand with that!

All new filters (including fletcher32) will be in a new PR!

[asinghvi17]

That's strange - tests are all passing, but the build failed when it came time to upload the codecov data, saying the job was already closed.

[asinghvi17]

Error: Bad response: 422 {"message":"Can't add a job to a build that is already closed. Build 10492471554 is closed. See docs.coveralls.io/parallel-builds","error":true}

(this line repeats in every log from the failed CI)

[meggart]

Is this ready to be merged? Or do you want to fix something in the CI pipeline first?

[asinghvi17]

Let's merge this, I'll fix the pipeline in another PR.