Create the output directory if it doesn't exist.
Fix up a parameter. Take into account that a user may want to specify a nested directory for which we would need to create multiple levels.
Only create the directory on processor 0.
Write warning message to std::cout instead of std::cerr.
Merge pull request #90 from bangerth/create-output-dir-2

Create output dir 2
Merge branch 'master' of https://github.com/geodynamics/aspect