tools: Time parameters like `4.d` get parsed as double and fail schema validation

Description of the bug

Using e.g. --max_time 4.d throws a validation exception because the schema expects a string for --max_time which should be granted by internal conversion of nextflow Duration objects to string, which seems to fail in this case

Steps to reproduce

Running a pipeline with the new parameter validation and a time parameter as described above should reproduce the bug. I did it with a fresh template.