Work around HydroTrend file length issue
Work around HydroTrend file length issue

HydroTrend exits with 'Abort Trap 6' if its `--in-dir` or
`--out-dir` is more than 80 characters. Dakota knows where its
current directly resides on the file system, so I think I can
safely use relative paths -- `../HYDRO_IN` and `../HYDRO_OUTPUT`.

mdpiper/dakota-experiments,mdpiper/dakota-experiments,mdpiper/dakota-experiments