Add function to save/load a list or dict of DataFrames to HDF5

Some usage like:

``` python
def save_data(dict_of_dataframes):
    for name, df in cwdata.items():
        df.to_hdf("data.h5", name)


def load_data():
    dict_of_dataframes = {}
    for n, coord in enumerate(coord_list):
        dict_of_dataframes[n] = pd.read_hdf("data.h5", str(n))
    return dict_of_dataframes
```
