Fix from_source called after importing GRIBReader

[sandorkertesz]

Fixes #375

Please note that GRIBReader now has to be imported as:

from earthkit.data.readers.grib.file import GRIBReader

However, using this internal object in any user code is discouraged.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.63%. Comparing base (680ddec) to head (aa3e933).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #380   +/-   ##
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  Coverage    89.63%   89.63%           
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  Files          117      117           
  Lines         7371     7371           
  Branches       644      644           
========================================
  Hits          6607     6607           
  Misses         636      636           
  Partials       128      128           

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