Support XQuery transforms

[DavidLeoni]

User might want to do some transformation on input XML prior import. There are many tools around to do just that, so we can limit ourselves to include in the distribution a ready to use solution with some docs on how to get started.

In particular, we can suggest using standard XQuery transformations with the included BaseX, which is an XML db that in theory should support transformations even on XMLs that don't fit in memory.

[DavidLeoni]

To keep things simple, for now we just

1. tell users to install separately BaseX and write some guidelines in the docs: https://github.com/diversicon-kb/divercli/blob/master/docs/Tools.md
2. provide in the released zip some useful xquery script , like rename-ids.xql