Add tesselated volumes (STL)

[GitFuchs]

This enables GATE10 to create a tesselated volume shape based on an Standard Triangle Language (STL) data file. Such a file contains a mesh of triangles for one object. It is a typical output format of Computer Aided Design (CAD) software. To create such a volume add a volume of type “Tesselated”. Please keep in mind, that no material information is provided, it has to be specified by the user. A Tesselated volume inherits the the same basic options as other solids described above such as translation or rotation.

Test and Documentation have been created.

Many thanks to David Sarrut for his help!

RESOLVES #240