Add various display types for pivot table such as Excel
Excel offers several types of values display for a pivot table (ratio of row, ratio of column, ratio of total, difference with, cumulated sum ...)
This ticket aims to implement this option in the pivot table API. I implemented the three types cited above for a start.
I chose "display_value" for the argument name. One can find too generic, so i'm opened to suggestions.

I think this could be better in the cookbook. Closing. But if you want to put an example there would be gr8.
 If you want to display values as ratio of column/row/table total like Excel/Calc allows, here's a piece of code which will do the job:

```
if display_value == "col_ratio":
    # Display values as a ratio of column total
    table = table.div(table.sum(axis=0), axis=1)
elif display_value == "row_ratio":
    # Display values as a ratio of row total
    table = table.div(table.sum(axis=1), axis=0)
elif display_value == "total_ratio":
    # Display values as a ratio of table total
    table = table.div(table.sum().sum())
```
 Indeed, I think this would be a nice example to add to the docs.

@gtnx In general, if you want to prevent the fact that we close a PR, it's better to first open an issue asking if such an enhancment would be desired.
 You're right. I realized it after i did the dev. I will do that next time
