Add custom x3ml:uuid xpath function.

[aindlq]

In MIDAS mappings for PHAROS we have a situation where if some ID element is defined then we want to use it's value as part of URI or otherwise generate UUID based on number of elements.

Without this change the whole mapping needs to be duplicated with different generators based on exist/not exist of that element.

With this change we can simply do something like:

(if (a2606) then a2606 else x3ml:uuid(lower-case(concat(a2700, a2690, a2664))))

and if a2606 is defined we get work/ID if it doesn't then we get work/UUID.