Add polytope tests for getting grib as a stream

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 12 lines in your changes missing coverage. Please review.

Project coverage is 90.58%. Comparing base (7fcae65) to head (327ab48).

Files with missing lines	Patch %	Lines
tests/sources/test_polytope.py	33.33%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #485 +/- ## =========================================== - Coverage 90.68% 90.58% -0.10% =========================================== Files 142 142 Lines 9739 9755 +16 Branches 459 460 +1 =========================================== + Hits 8832 8837 +5 - Misses 751 762 +11 Partials 156 156 ```

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