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`.

Inst_eddies/Analysis/plot_eddy_contours.py

@@ -27,7 +27,6 @@ from os import path
 from matplotlib import patches
 import f90nml
 import sys
-sys.path.append(path.join(sys.path[0], "../../Common"))
 import util_eddies
 
 def select_ishapes(d, handler, window = None):

Inst_eddies/Analysis/util_eddies.py

+../../Common/util_eddies.py
\ No newline at end of file