Improved flavor check and added access to slc-rel Github data

[tomuben]

Background

Flavor check

When we create a new instance of class ScriptLanguagesContainer we need to ensure that the given flavor exists in the cloned repository. The current implementation might raise an error message which is confusing to the user, because the notebook can have set an invalid flavor name for the currently used slc release.

Add constant GITHUB_URL to ScriptLanguageContainer

The notebooks should display which current version of the Script-Languages-Release repository is being used.

Add method to list all available flavors for Script-Languages-Release version

Notebooks need to display all available flavors for the currently used tag of script-languages-release.

Acceptance Criteria

1. ScriptLanguagesContainer.create should raise an exception with meaningful error message if the given flavor does not exist.
2. Add constant ScriptLanguageContainer.GITHUB_URL
3. Add a function ScriptLanguageContainer.list_available_flavors()