Allow overriding of part of the file-tree

Override some of the templates in one file-tree with that of another file-tree:

new_filetree = filetree.override(user_filetree, required=[...], optional=[...])

This overrides templates in filetree with values from user_filetree. They will be added to the filetree even if they do not exist beforehand.

You should also supply a list of template keys using one of two keywords:

• required=[...]: these are keywords are required in the user_filetree. If any of these keys are missing from required an error is raised.
• optional=[...]: these are keywords that can be overwritten. If they are provided in user_filetree, they will be. Otherwise the original values in filetree will be used.

This can be implemented using the existing add_template.