Added the chance to use more than one controlled vocabulary.

XML Schema used to describe the models has been updated in order to
be able to tell that a column can have valid values from more than
one vocabulary.
BP::Model now deals with restrictions of multiple CVs

Although the XML schema allowed to declare more than one controlled
vocabulary as a restriction for a given column, BP::Model and the
tools using it (SQL generators and so) where not prepared for such
feature.
Merge branch 'develop' of https://github.com/inab/BP-Schema-tools into develop

Conflicts:
	model-doc-generator.pl
Merge branch 'develop' of https://github.com/inab/BP-Schema-tools into develop

Conflicts:
	lib/BP/Model.pm