Pandas panel deprecation will remove I/O functionality to HDF5/excel
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
import numpy as np

data = np.random.rand(3,3,3)
major_axis = ['maj_ax_{}'.format(i) for i in range(3) ]
minor_axis = ['min_ax_{}'.format(i) for i in range(3) ]
items = ['item_{}'.format(i) for i in range(3) ]

pnl = pd.Panel(
    data=data,
    major_axis=major_axis,
    minor_axis=minor_axis,
    items=items
)

pnl.to_hdf('test.h5', key='df')

reread_data = pd.read_hdf('test.h5')

reread_data.to_excel('data.xlsx')

```
#### Problem description

With the Panel class being deprecated and is going to be removed in the future, exporting and reading 3D data to and from an HDF5 file will be lost, excel I/O for 3D data as well. At the moment, Xarray that is proposed as a replacement for handling 3D data does not provide this functionality.

#### Expected Output
An HDF5 and excel file that contains 3D data.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 61 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.2
pytest: 2.8.5
pip: 9.0.1
setuptools: 20.7.0
Cython: 0.23.4
numpy: 1.12.1
scipy: 0.19.0
xarray: 0.9.6
IPython: 5.1.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: 0.999
sqlalchemy: 1.0.11
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: 0.0.7
pandas_gbq: None
pandas_datareader: None
</details>
