BUG: convert_objects on > 2ndim raises
```
In [40]: x = Panel(dict(A = dict(a = ['1','1.0'])))

In [41]: x
Out[41]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 1 (items) x 2 (major_axis) x 1 (minor_axis)
Items axis: A to A
Major_axis axis: 0 to 1
Minor_axis axis: a to a

In [42]: x.values
Out[42]: 
array([[['1'],
        ['1.0']]], dtype=object)

In [43]: x.convert_objects(convert_numeric='force')
ValueError: Buffer has wrong number of dimensions (expected 1, got 2)
```
