add support for parent/child in xslt

[ferrighi]

Add support for parent and child in the following:

• [x] mmd-to-dif.xsl
• [x] mmd-to-dif10.xsl
• [x] mmd-to-inspire.xsl
• [x] mmd-to-wmo.xsl

This will support convey the available information also through CSW.