Replace _root() with _head()
Add _tail function
Get rid of modify_expected_filename methods
Remove hashfilenames processor, it's now included in the output node
Bundle needs to override prepare() because it changes the filename
By default, processors do not modify inputs at all for prepare(), so just return the inputs
Sorry for the monster-commit, rename _head() and _tail() to head and tail, jiggled around the entire processing logic to now use the three steps prepare, is_dirty, run
Fix some broken