Cleaning up head=None keyword usage.

This commit removes many unused head=None keywords.

MODEL: the original idea behind the head=None keyword was a sort of dynamic
indicator of "where the score manager is focused right now." So if the score
manager were focused on the Red Example Score, then head='red_example_score'
would be set and passed into various methods. Likewise, if the score manager
were focused on the material library then head=None would be set (indicating
that the score manager is outside of any one particular score) or perhaps
head='material library' would be set (allowing for a symbolic value).

MODEL: as the model has developed, the use of the head=None keyword has been
replaced by session variables like current_score_package_directory_path and
current_score_snake_case_name.

MODEL: the coming series of commits will remove as many occurrences of the
head=None keyword. If it is found necessary to preserve at least some of the
occurrences of the keyword, then all remaining instances will be changed to
pass (filesystem) paths rather than packages.