Add option to persist handle to NetCDF files

[meggart]

This can significantly help to speed up opening large multi-file netcdf datasets.

[codecov]

Codecov Report

Attention: Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.

Project coverage is 62.06%. Comparing base (c9d7d6d) to head (74c8716).
Report is 32 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/NetCDFExt.jl	87.50%	3 Missing ⚠️
src/datasets/datasetinterface.jl	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #31       +/-   ##
===========================================
+ Coverage   51.75%   62.06%   +10.31%     
===========================================
  Files          13       12        -1     
  Lines         342      377       +35     
===========================================
+ Hits          177      234       +57     
+ Misses        165      143       -22     

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