langevin: Define DROPSUFFIX in localconf.mk
split: Some amendments related to splitdir

- Ensure to never select absolute path /splitdata.
- Make comments in standalone more comprehensible.
- Remove redundant assignment in downsampling.
- Show splitdir with showdata also in correlation.
common: Avoid circular dependency for linked data

Allow . in datadirs to use ./splitdata as splitdir
but do not search data files for linking there.