Add serialisation to grib fieldlists

[sandorkertesz]

Relates to #453
This PR adds serialisation to the following objects:

GRIBReader
GribMaskFieldList
GribMultiFieldList
GribFieldListInMemory

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.40%. Comparing base (809daee) to head (e076386).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #474      +/-   ##
===========================================
+ Coverage    90.33%   90.40%   +0.07%     
===========================================
  Files          128      128              
  Lines         8832     8899      +67     
  Branches       757      764       +7     
===========================================
+ Hits          7978     8045      +67     
  Misses         707      707              
  Partials       147      147              

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