Investigate common Java tools

[fgeorges]

Someone needs to propose something/work on common Java tools for
extensions, and analyse and describe differences between Adam's and
Florent's existing work.

More context, from the minutes of the meeting in Prague in Feb. 2015:
https://lists.w3.org/Archives/Public/public-expath/2015Feb/0005.html

The group expressed interest for common Java tools to write
extensions. A common data model to use in extension functions, a
common API to declare extension functions. That would enable one to
write extension functions in Java in a generic way (or at least enable
reusing as much as possible between several implementations).

There has been some independent work on this area by both Adam and
Florent, available at:

• https://github.com/fgeorges/expath-file-java
• https://github.com/fgeorges/expath-file-saxon
• https://github.com/exquery/exquery/tree/xdm-model/xdm
• https://github.com/exquery/exquery/blob/master/expath-file-module/src/main/scala/org/exquery/expath/module/file/FileModule.scala#L54
• https://github.com/adamretter/exist-expath-file-module/blob/master/src/main/scala/org/exist/expath/module/file/FileModule.scala