ENH: Prettier printing of DataFrames with nested objects, discuss?
I frequently place objects which are not strings or numbers in my DataFrames.  I've read many Stack Overflow comments which discourage this, and I know that it general it should be avoided, but I believe that I have a valid use case.  If the objects are iterable (like another DataFrame), I protect them from Pandas attempt to broadcast the elements by wrapping the object in a single-element list.  This all works, but it doesn't print nicely.  

I looked into `pandas.set_option()` to see whether there was a way to clean up printing in the way that I wanted.  When I didn't find it, I wrote a work-around.  I defined a PrettyDataFrame subclass which has only one method, `__str__()`.  When a column's dtype is `"object"`, I assume that object is a list, and I print the dtype of element zero.  A typical output from an IPython interpreter session looks like this:

```
In [3]: exp
Out[3]: 
   embed_dim  kernel_size  min_loss  best_epoch  elapsed_time            history    embedding best_weights
0          1            3 -0.717174          32    361.163418  <PrettyDataFrame>  <DataFrame>     <Series>
1          1            3 -0.717349          34    424.519406  <PrettyDataFrame>  <DataFrame>     <Series>
2          1            3 -0.717181          25    287.420010  <PrettyDataFrame>  <DataFrame>     <Series>
3          1            3 -0.717192          31    327.283332  <PrettyDataFrame>  <DataFrame>     <Series>

In [4]: exp.loc[2].history
Out[4]: 
   initial_epoch  final_epoch  min_loss  best_epoch best_weights       loss
0              0           19 -0.717125          19       <list>  <ndarray>
1             20           28 -0.717181          25       <list>  <ndarray>
```

As the Pandas GitHub guide recommends, [I posted to Stack Overflow first, asking whether the feature I wanted exists](https://stackoverflow.com/questions/61184181/pretty-printing-pandas-dataframes-with-complex-objects-inside-individual-cells).  After waiting for several days and receiving no replies, I'm moving here to continue the discussion.

I'm not ready to submit a pull request because what I wrote is rather specific to my needs.  I don't know if anyone else would find this useful, or if they would want it to look different.  Additionally, if this kind of pretty-printing was part of a standard DataFrame instead of a weird subclass, the printing behavior would presumably have to be configurable through set_option(), and I haven't begun to think about how to define that.

Looking forward to any discussions.  Thanks.