Switch to a newer XSLT processor supporting at least XSLT 2.0 and XPATH 2.0

[RKrahl]

In the current release version icat.oaipmh uses the XSLT processor from the Java standard library (the javax.xml.transform.* classes). The problem is that this only supports XSLT 1.0 and XPATH 1.0, which makes many things in the XSLT files needed to be provided by the configuration really cumbersome. We should switch to a third party library supporting more recent XSLT and XPATH versions.

The 2.0 versions would already make things much easier. But it seems that there is only one free implementation available anyway: Saxon HE, which supports the newest versions XSLT 3.0 and XPATH 3.1. (The corresponding maven pom.xml snippet.)