title: Multidimensional arrays in Pandas columns
body: <p>I have data that will consist of a number of attributes which may be described by arrays of arbitrary length (eg., an object can contain some number of clusters and I want to store the sizes of each constituent cluster as a column, but the number of clusters per object can range from 0 to \infty, in principle). Is there a way to support arrays of any length as column data in a Pandas dataframe? I realize I could use a panel, but AFAIK one would need to know the depth of the panel (which in principle I can't know until I've loaded the data) and in addition the panel may be very sparse since in the example, many objects may have only very few clusters. </p>
  
  <p>If I just use a numpy array with dtype=object, will there be any implications for storing in H5Store or in the performance of Pandas selections or anything else?</p>