Ownership

[magnusuMET]

This is a complete revamp of ownership and caching of the file. The rust side is now not reading more than strictly necessary, and instead goes through netcdf-c. The ownership is now determined by PhantomData, which prevents early dropping of the File.

Multiple VariableMut can be held simultaneously (from different groups or from variables_mut), with the PhantomData preventing data races. This change should decrease memory usage of the library, especially when not reading all attributes of a file, as only netCDF-c holds the metadata (fixes #44).

The library might be a bit more annoying to use, as almost all operations can fail in the netcdf-c layer. Some unwrapping can probably be done on the library side, so application users can deal with an easier API. I am leaving this as a future exercise.

[codecov-io]

Codecov Report

Merging #51 into master will decrease coverage by 2.13%.
The diff coverage is 86.51%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   86.87%   84.74%   -2.14%     
==========================================
  Files           7        7              
  Lines        1280     1278       -2     
==========================================
- Hits         1112     1083      -29     
- Misses        168      195      +27

Impacted Files	Coverage Δ
src/attribute.rs	89.26% <100%> (+0.08%)	⬆️
src/error.rs	44% <35.71%> (-3.5%)	⬇️
src/group.rs	82.55% <82.5%> (-8.22%)	⬇️
src/file.rs	79.8% <85.88%> (-11.65%)	⬇️
src/variable.rs	83.95% <86.84%> (+0.18%)	⬆️
src/lib.rs	93.75% <91.66%> (-6.25%)	⬇️
src/dimension.rs	96.73% <97.56%> (+0.73%)	⬆️

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