Make `cfdm.read` always return unicode strings from netCDF files

[davidhassell]

Fixes #251

[codecov[bot]]

Codecov Report

Merging #252 (0e8dfcf) into main (268693c) will decrease coverage by 0.01%. The diff coverage is 88.89%.

:exclamation: Current head 0e8dfcf differs from pull request most recent head 8cd0a4f. Consider uploading reports for the commit 8cd0a4f to get more accurate results

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
- Coverage   87.54%   87.52%   -0.01%     
==========================================
  Files         124      124              
  Lines       12804    12804              
==========================================
- Hits        11208    11206       -2     
- Misses       1596     1598       +2     

Flag	Coverage Δ
unittests	87.52% <88.89%> (-0.01%)	:arrow_down:

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Impacted Files	Coverage Δ
cfdm/data/netcdfarray.py	68.30% <83.34%> (ø)
cfdm/examplefield.py	100.00% <100.00%> (ø)
cfdm/read_write/netcdf/netcdfread.py	83.88% <100.00%> (-0.04%)	:arrow_down:

... and 2 files with indirect coverage changes

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[sadielbartholomew]

@davidhassell are you looking for a review on this?