Fix usage when trollsched is not installed

[pnuu]

The check_valid_data_fraction() doesn't have a check whether pytroll-schedule is installed or not.

This PR adds the check, and makes the get_scene_coverage() helper function a private function to show it should not be called directly.

• Closes The check_valid_data_fraction plugin does not check for pytroll-schedule availability #222
• Tests updated
• Tests passed
• Passes flake8 trollflow2

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.99%. Comparing base (e0d795e) to head (49bafdc).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   95.96%   95.99%   +0.02%     
==========================================
  Files          14       14              
  Lines        3272     3293      +21     
==========================================
+ Hits         3140     3161      +21     
  Misses        132      132              

Flag	Coverage Δ
unittests	95.99% <100.00%> (+0.02%)	⬆️

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[gerritholl]

Please add a unit test for the added lines if possible.

[mraspaud]

LGTM! thanks for fixing this check and making the code better along the way!