Better error message for invalid comparisons

when you compare something like
@invalid > 3
we throw an error message instead of letting
the assert(false) Lua function catch it.
Change indentation to 4 spaces

Only whitespace changes. It is really nicer on
my notebook with small font size and sublime text
editor.
Bugfix: incorrect columnwidth calculation

We have to find the width between two _glues_ not
between two _glyphs_

Other change: fix return type of broken comparison
Enhance Schema

align and such can contain {...} XPath expressions
Allow barcodes in tables

You can now use <Td><Barcode ... /></Td>