PERF: large subset of block cast
from [SO](http://stackoverflow.com/questions/37845884/pandas-setting-column-subset-slow)

I'm guessing there's a lot of inference / checks along this path, but this is slower than I'd expect.

``` python
In [66]: df = pd.DataFrame(np.random.randint(0,1000, size=(10000, 1026)))
    ...: f = list(range(1024))
    ...: 

In [67]: %time df[f] = df[f].astype(float)
Wall time: 10.4 s
```
