Auto initialize in `startproc`

[nhz2]

Fixes #70. Third time's the charm

This PR avoids the complexity of interacting with GC in #71 by checking if the context is null when startproc is called. If the context is null startproc initializes the context.

This means finalize is still needed to prevent memory leaks, but there is no issue of use after free.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 61.35%. Comparing base (e7edfed) to head (60079dd). Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/compression.jl	70.00%	3 Missing :warning:
src/decompression.jl	70.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   61.55%   61.35%   -0.21%     
==========================================
  Files           5        5              
  Lines         372      370       -2     
==========================================
- Hits          229      227       -2     
  Misses        143      143              

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[nhz2]

Interesting approach. We may still want to consider the finalizer.

Yes, having a finalizer should be compatible with this PR, and would also prevent memory leaks.

[nhz2]

@mkitti Could you review this PR again, I think I addressed your comments.

[mkitti]

Overall, it looks fine. I will try to look more closely tonight.

[ali-ramadhan]

Thank you @nhz2 and @mkitti! This fixes https://github.com/JuliaIO/JLD2.jl/issues/599 which will allow us to use zstd to compress Oceananigans.jl outputs.

Would it be possible to tag a new version of CodecZstd.jl?