bugfix call to add VTUs in `generate_benchmark_script()`
When executing a generate_benchmark_scripts() to create a R script for an existing ogs6 PRJ file, the call to add VTU does fail.
Announcement: OGS Community Meeting 2022 in September
When executing a generate_benchmark_scripts() to create a R script for an existing ogs6 PRJ file, the call to add VTU does fail.