Should be able to share subautomatons between several Modex rules files

Currently, the _use_ keyword allows to list classes in external files to define gazeteers and the _include_ keyword allows to compile rules from files external to the current rules file.

Also, it is possible to define subautomatons in a file that are used in several rules of this file.

But it is not possible to share subautomatons between rules files. This would be useful to avoid duplication and help the maintenance of rules files. In this case, error reporting should take this inclusion into account.