Guard plot_cross_validation.save_outliers for institutionId and seriesId columns confirm with unit test

[dumbledad]

Fixing Issue #423

Overview:

Guarding the call to create_portal_query_for_outliers so that the required columns are present, and providing unit test that confirms the change.

Detail:

The functions save_outliers and create_portal_query_for_outliers in InnerEye.ML.cisualizers.lot_cross_validation
fail if the columns institutionId and seriesId are missing from the data. This PR

• Augments create_portal_query_for_outliers so that it checks for the required columns and raises an informative ValueError if they are missing. The function description is updated accordingly.
• Changes save_outliers in two ways. If the columns are missing
  • they are not used in the groupby which gleans the stats, and
  • the portal query is omitted.
• Test data is provided to check that the revised save_outliers produces expected data when the columns are missing
• The unit tests test_save_outliers and test_create_portal_query_for_outliers in Tests.ML.visualizers.test_plot_cross_validation are extended to exercise the new code with columns missing.

Checklist:

• Ensure that your PR is small, and implements one change.
• Add unit tests for all functions that you introduced or modified.
• Run PyCharm's code cleanup tools on your Python files. (I'm using VS Code and have not done this.)
• Link the correct GitHub issue for tracking.
• Update the Changelog file: Describe your change in terms of
  Added/Changed/Removed/... in the "Upcoming" section.
• When merging your PR, replace the default merge message with a description of your PR,
  and if needed a motivation why that change was required.