Make ngs_mapping step parts create output links

[holtgrewe]

Is your feature request related to a problem? Please describe.
Currently, we generate the links to output in a step part of the workflow. This has the disadvantage that there is one snakemake rule execution per file. This is very inefficient.

Describe the solution you'd like
The step parts should do this themselves. Also, we can then delegate generation of result files for the workflow to the step parts.

Describe alternatives you've considered
N/A

Additional context
N/A