Decide defaults for RTReadWalker

[magicDGS]

After the #441 (implemented in #464), we should decide:

• Default filters - currently the one from GATK (WellformedReadFilter) is the only available.
• Transformers - currently the one for checking quality is the one used.
• Arguments - currently no default argument exclusive for ReadTools is included

Maybe no default should be added, but it might be useful to have an argument collection with defaults to share between all ReadTools walker classes.