allow lb in div elements

I had lb in div elements in TEI (as we used div instead of p elements for separating content).  These were deleted by common/common_core.xsl.  This patch restores lb in div elements, and allows the individual format handlers to deal with them.  For LaTeX, this did what I wanted.

I guess there must have been a reason to delete them, but I can't imagine one.  If there is an issue with this,  maybe this calls for a configuration option?