* getter.xsl added

* getter.xsl added
  In this stylesheet there is a simple function to make the URL
  of an XML file given its URI. (The URL is used to ask the getter
  to retrieve the file)
* every other stylesheet (but ricerca.xsl that is unused and
  links_library.xsl) has been changed to use the new function
  instead of computing the URL cutting&pasting the same code
  again and again.