Process multiple notebooks

[mwermelinger]

Add a function like setup([nb1, nb2, ...]) that takes all notebook filenames in order and adds to metadata info about folder and file, previous and next notebook, so that subsequent functions can process each notebook individually in context, e.g. to add navigation links.

The argument probably should be nested list to show part/chapter/section hierarchy so that 'up' navigation links can be added to lower level notebooks and TOCs can be added to higher level notebooks.