update paths to ogs6-BM files in ensemble vignette

actual paths refer to examples/Theis_problem/benchmark_files and examples/Theis_well_pumping/benchmark_files. However, these path are not exported. Anyhow the BM files should go into inst/extdata/benchmarks.

Tasks:

• move BM-files into inst/extdata/benchmarks
• update ensemble vignette