refactor: Singularity from conda environment might cause errors

The current APAeval conda environment contains Singularity:
https://github.com/iRNA-COSI/APAeval/blob/d4e7c142185f5e8ce45b94ee66510a3cc665a9dd/apaeval_env.yaml#L15

For some reason, e.g. no root access, the Singularity version from conda **can** lead to execution errors.

This can fixed quite easily though:
- Manually remove Singularity from this conda environment and install/use an own version independently. This fix should then be highlighted by an additional note in the install instructions.
- Or remove Singularity from the conda environment (i.e. line 15 in `apaeval_env.yaml`) and require independent installation thereof. Again, this should be reflected in the documentation.

The first fix might be preferred if the error occurs rarely and the second one if it occurs often.