Implement repr to ArrayField

[sandorkertesz]

This PR implements the repr for an ArrayField. E.g. if f contains surface data:

print(f)
>>> ArrayField(2t,None,20200513,1200,0,0)

Note: this repr is GRIB specific.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.56%. Comparing base (59fc21b) to head (c179576).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #455   +/-   ##
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  Coverage    90.56%   90.56%           
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  Files          123      123           
  Lines         8329     8333    +4     
  Branches       701      701           
========================================
+ Hits          7543     7547    +4     
  Misses         655      655           
  Partials       131      131           

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