decode-html: decode only when string contains "<"
XPath: comparing a nil value should return false

Non existent attributes are equal to "empty set",
and anything compared to empty set is false
(hopefully I've got that right).
Schema cleanup, allow @data in <Tr>

Attribute data is used for dynamic header/footer
Colorbar: default color on empty string

The empty color ("") and the nil color now
default to 'black'
Allow multiple elements in ForAll
Remove non-table elements from table

We can mix <SetVariable> and <Tr> in <Tables>,
which is useful in the body of <ForAll>. To make
the <Table> code work, we must remove all the
unnecessary elements.
New default color 'black'

One day we should have more default colors such
as the CSS colors.
Add profiling function (disabled)

Just use the other main loop in publisher.tex
to activate the profiler.