fix bug extruded volume: 
The parser wanted to create extruded volume begining systematically from 1. 
Thus, when there already existed volumes with number 1,2,3,etc, extruded volumes was created.


git-svn-id: https://geuz.org/svn/gmsh/trunk@19530 59c8cabf-10d6-4a28-b43f-3936b85956c4