PERF: mutli-index selection vs repeated selections
from [SO](http://stackoverflow.com/questions/30657115/optimize-pandas-query-on-multiple-columns-multiindex/30667394#30667394)

```
idx = pd.IndexSlice

n=10000000
np.random.seed(1234)
mdt = pandas.DataFrame()
mdt['A'] = np.random.choice(range(10000,45000,1000), n)
mdt['B'] = np.random.choice(range(10,400), n)
mdt['C'] = np.random.choice(range(1,150), n)
mdt['D'] = np.random.choice(range(10000,45000), n)
mdt['x'] = np.random.choice(range(400), n)
mdt['y'] = np.random.choice(range(25), n)


test_A = 25000
test_B = 25
test_C = 40
test_D = 35000

eps_A = 5000
eps_B = 5
eps_C = 5
eps_D = 5000

mdt2 = mdt.set_index(['A','B','C','D']).sortlevel()
```

selection

```
    In [106]: %timeit  mdt2.loc[idx[test_A-eps_A:test_A+eps_A,test_B-eps_B:test_B+eps_B,test_C-eps_C:test_C+eps_C,test_D-eps_D:test_D+eps_D],:]
    1 loops, best of 3: 4.34 s per loop
```

Repeated selection

```
    In [105]: %timeit mdt2.loc[idx[test_A-eps_A:test_A+eps_A],:].loc[idx[:,test_B-eps_B:test_B+eps_B],:].loc[idx[:,:,test_C-eps_C:test_C+eps_C],:].loc[idx[:,:,:,test_D-eps_D:test_D+eps_D],:]
    10 loops, best of 3: 140 ms per loop

```
