- Add copyright headers to files that did not already have them
- Update the year to the last year the file was modified in files with
existing headers.
Previously the `once_per_instance` used the output of `__repr__` to
identify the class of where the decorated method was called from. For
classes that override this method to produce the same output for
different instances of the same class this caused different instances to
be mistakenly treated as one. Now use the hash of the containing class
instead of the string representation and update the tests to catch this
error.
Previously there was no differentiation between the same method at different
inheritance levels, therefore using the once_per_instance_decorator with super call would
prevent the super call from executing as they were considered the same
instance.
