GridGenerator::flatten_triangulation.

Added GridTools::flatten_triangulation + one test.

Fixed WB comments.

Moved flatten triangulation to GridGenerator.

Fixed TH and WB comments.

Fixed indentation issue.
Merge pull request #95 from luca-heltai/flatten_triangulation

GridGenerator::flatten_triangulation: Gives the ability to copy a flattened version of a Triangulation, with possibly different spacedims.