Use a symbolic link

Use a symbolic link instead of appending to `sys.path`. THe motivation
is that in the Python interpreter, the following statements failed
before the present commit:

```python
import sys
sys.path.append("/path/to/Detection_eddies/Inst_eddies/Analysis")
import plot_eddy_contours
```

`sys.path[0]` then does not contain
`/path/to/Detection_eddies/Inst_eddies/Analysis` so
`plot_eddy_contours` could not find `util_eddies`.