API: raise/warn SettingWithCopy when chained assignment is detected
```
In [1]: df = DataFrame({"A": [1, 2, 3, 4, 5], "B": [3.125, 4.12, 3.1, 6.2, 7.]})

In [2]: row = df.loc[0]
```

Default is to warn

```
In [3]: row["A"] = 0
pandas/core/generic.py:1001: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
  warnings.warn(t,SettingWithCopyWarning)

In [4]: row
Out[4]: 
A    0.000
B    3.125
Name: 0, dtype: float64

In [5]: df
Out[5]: 
   A      B
0  1  3.125
1  2  4.120
2  3  3.100
3  4  6.200
4  5  7.000
```

You can turn it off

```
In [6]: pd.set_option('chained',None)

In [7]: row["A"] = 0

In [8]: row
Out[8]: 
A    0.000
B    3.125
Name: 0, dtype: float64

In [9]: df
Out[9]: 
   A      B
0  1  3.125
1  2  4.120
2  3  3.100
3  4  6.200
4  5  7.000
```

Or set to raise

```
In [10]: row["A"] = 0
SettingWithCopyError: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead
```
