missing .value in CactusField.py line 167

Dear team,

I build CACTUS recently and tried the CACTUS-tools. I found a copy/paste/type bug in CactusField.py line 167.

```
-      # extract columns
-       x   = df.loc[:, x_col_name]
```

```
+      # extract columns
+      x   = df.loc[:, x_col_name].value
```

Thanks,
Johannes