Multi-Index Dataframes do not cleanly support dimension-reducing arithmetic operations
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> df = pd.DataFrame(data = [[1,2,3,4],[5,6,7,8]], index = [0,1], columns = pd.MultiIndex.from_product([('a0','a1'),('b0','b1')]))
>>> df.loc[:,idx[:,'b0']]
  a0 a1
  b0 b0
0  1  3
1  5  7
>>> df.loc[:,idx[:,'b1']]
  a0 a1
  b1 b1
0  2  4
1  6  8
>>> df.loc[:,idx[:,'b0']] * df.loc[:,idx[:,'b1']]
   a0      a1    
   b0  b1  b0  b1
0 NaN NaN NaN NaN
1 NaN NaN NaN NaN
```

#### Problem description

Since Panel became deprecated, I am having to do a _lot_ of multi-dimensional array arithmetic using multi-index DataFrames, but the multi-index does not support simple reductive arithmetic (any operation that should return a lower-dimensional answer), because all operations return the original index and columns.

If any of the arithmetic functionality of the old Panel is to be retained by the Multi-Index Dataframe, it seems that reductive operations should be natively supported. Alternatively, maybe somebody knows a clean workaround?

#### Expected Output
```python
>>> # Expected output would *something* similar to this workaround.....
>>> pd.DataFrame(df.loc[:,idx[:,'b0']].values * df.loc[:,idx[:,'b1']].values, index = [0,1], columns = pd.MultiIndex.from_product([['a0','a1'],['b0*b1']]))
     a0    a1
  b0*b1 b0*b1
0     2    12
1    30    56
>>> # .....Or maybe returning a level-reduced set of axes.
>>> pd.DataFrame(df.loc[:,idx[:,'b0']].values * df.loc[:,idx[:,'b1']].values, index = [0,1], columns = ['a0','a1'])
   a0  a1
0   2  12
1  30  56
```
#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here
24-Jul-17 14:21:05 DEBUG     lzma module is not available
24-Jul-17 14:21:05 DEBUG     Registered VCS backend: git
24-Jul-17 14:21:05 DEBUG     Registered VCS backend: hg
24-Jul-17 14:21:05 DEBUG     Config variable 'Py_DEBUG' is unset, Python ABI tag may be incorrect
24-Jul-17 14:21:05 DEBUG     Config variable 'WITH_PYMALLOC' is unset, Python ABI tag may be incorrect
24-Jul-17 14:21:05 DEBUG     Config variable 'Py_UNICODE_SIZE' is unset, Python ABI tag may be incorrect
24-Jul-17 14:21:05 DEBUG     Config variable 'Py_DEBUG' is unset, Python ABI tag may be incorrect
24-Jul-17 14:21:05 DEBUG     Config variable 'WITH_PYMALLOC' is unset, Python ABI tag may be incorrect
24-Jul-17 14:21:05 DEBUG     Config variable 'Py_UNICODE_SIZE' is unset, Python ABI tag may be incorrect
24-Jul-17 14:21:05 DEBUG     Registered VCS backend: svn
24-Jul-17 14:21:05 DEBUG     Registered VCS backend: bzr

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 5.3.0
sphinx: 1.5.6
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.3
bs4: 4.6.0
html5lib: 0.999
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
