Add substitution variable ${datafile_path}, similar to ${logfile_path}

[dbeyer]

There are two sets of result files that BenchExec produces:

• Log files (those results that the tool outputs to stdout)
• Data files (those results that the tool outputs to files on disk)

The former are stored in a directory "...logfiles/" and the latter are stored in a directory "...files/".

The former can be referred to using variable ${logfile_path} (see https://github.com/sosy-lab/benchexec/blob/master/doc/benchexec.md) but there is no variable ${datafile_path} for the latter.

[dbeyer]

I think this can be closed.

[PhilippWendler]

Hm, let's leave it open because it is a valid feature request.