GroupBy on DatetimeIndex with float32 values VERY slow
I have a DataFrame with a DatetimeIndex and two float32 columns.

``` ipython
In [35]: %timeit a.groupby(level=0).last()
1 loops, best of 3: 2.11 s per loop

In [36]: a = a.astype(np.float64)

In[37]: %timeit a.groupby(level=0).last()
1000 loops, best of 3: 911 us per loop
```

Either way, the result of the groupby is all float64s. I would lilke to preserve float32 dtypes if possible.

Also, there are other operations (resample, shift) that are also very slow on float32 data but I'm pretty sure this is related.
