artifactcache.py: API Cleanup - Add contains_ref() method

    # contains():

    #

    # Check whether the artifact for the specified Element is already available

    # in the local artifact cache.

    # Check whether the (project state) artifact of the specified Element is

    # already available in the local artifact cache.

    #

    # Args:

    #     element (Element): The Element to check

    #     key (str): The cache key to use

    #

    # Returns: True if the artifact is in the cache, False otherwise

    # Returns: True if the Element's (project state) artifact  is in the cache,

    # False otherwise

    #

    def contains(self, element, key):

        ref = self.get_artifact_fullname(element, key)

        return self.contains_ref(ref)

    # contains_ref():

    #

    # Check whether an artifact is already available in the local artifact cache.

    #

    # Args:

    #     ref (str): The ref to check

    #

    # Returns: True if the artifact is in the cache, False otherwise

    #

    def contains_ref(self, ref):

        return self.cas.contains(ref)

    # contains_subdir_artifact():