Fix very slow reads of datasets with compession by gathering or DSG ragged arrays

[davidhassell]

Fixes #260

Also includes the compression_index keyword parameter to NetCDFRead._create_data. This is not used by cfdm, yet, but will be by cf-python.

[codecov[bot]]

Codecov Report

Merging #261 (3046463) into main (4a71a4b) will increase coverage by 0.02%. The diff coverage is 100.00%.

:exclamation: Current head 3046463 differs from pull request most recent head 1c0b78d. Consider uploading reports for the commit 1c0b78d to get more accurate results

@@            Coverage Diff             @@
##             main     #261      +/-   ##
==========================================
+ Coverage   87.29%   87.30%   +0.02%     
==========================================
  Files         125      125              
  Lines       12873    12879       +6     
==========================================
+ Hits        11236    11243       +7     
+ Misses       1637     1636       -1     

Flag	Coverage Δ
unittests	87.30% <100.00%> (+0.02%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cfdm/cfdmimplementation.py	83.02% <ø> (ø)
cfdm/data/data.py	88.99% <100.00%> (+0.10%)	:arrow_up:
cfdm/read_write/netcdf/netcdfread.py	83.26% <100.00%> (+0.05%)	:arrow_up:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more

[davidhassell]

Thanks, Sadie.