BUG: Fix Panel instance variable namespace issue GH3440
The change is minor but so easy I thought I'd submit it: If you switch the list comprehensions to generators, it sidesteps leaking the temporary variables into the class variable scope.

This will be microscopically slower, but they are just class variables so it really doesn't matter.
fixes #3440

https://travis-ci.org/danbirken/pandas/builds/9836459

```
import numpy as np
from pandas import DataFrame, Panel

df0 = DataFrame(np.zeros([3,4]))
df1 = DataFrame(np.ones([3,4]))

p = Panel({'a':df0, 'b':df1})

In [1]: p.a
Out[1]:
   0  1  2  3
0  0  0  0  0
1  0  0  0  0
2  0  0  0  0

In [2]: p.b
Out[2]:
   0  1  2  3
0  1  1  1  1
1  1  1  1  1
2  1  1  1  1
```

@danbirken can u add a test? thx
 There you go

Travis: https://travis-ci.org/danbirken/pandas/builds/9843789
 @danbirken sweet thanks!
 @jreback will this be fixed in #3482?
 I think this fix is fine
 @danbirken thanks!
