add support for variable-length strings

[ilia-kats]

This uses the vlen-utf8 filter, which is one of the standard filters included with Python-Zarr.

[coveralls]

Pull Request Test Coverage Report for Build 4819715507

• 26 of 30 (86.67%) changed or added relevant lines in 3 files are covered.
• 14 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.4%) to 85.012%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Filters.jl	22	23	95.65%
src/ZArray.jl	2	3	66.67%
src/metadata.jl	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/ZArray.jl	2	94.29%
src/MaxLengthStrings.jl	5	33.33%
src/metadata.jl	7	78.49%

Totals
Change from base Build 4698045635:	-1.4%
Covered Lines:	726
Relevant Lines:	854

---

💛 - Coveralls

[meggart]

Thanks a lot for preparing this PR. It would be really nice to have VLenUTF8 support in here. I think technically the PR would be breaking because dumping a string vector into a zarr array would result into a different representation on disk as before. So I would make sure bunp the minor version in this PR to not forget.

[meggart]

I knwo this was a quite ad-hoc definition of SenMissArray, probably better to keep the sentinel value in a struct field instead of a type parameter. Now that the idea was taken up also here: https://github.com/JuliaData/SentinelArrays.jl it might be an option to just use that implementation. This should not stop this PR from being merged though.

[coveralls]

Pull Request Test Coverage Report for Build 4819746815

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

• 27 of 32 (84.38%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 85.814%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Filters.jl	22	23	95.65%
src/ZArray.jl	3	5	60.0%
src/metadata.jl	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
src/MaxLengthStrings.jl	5	33.33%

Totals
Change from base Build 4698045635:	-0.6%
Covered Lines:	738
Relevant Lines:	860

---

💛 - Coveralls