Assignment 3 Q-12 and Q-13

Thanks for your contribution but I suggest that:

In Q-13:
```
Top15['PopEst'] = Top15['PopEst'].apply(lambda x: "{:,}".format(x))
```
this will adding commas "," to thousands

in Q-12:
```
x = Top15.groupby(['Continent','bins']).size()
x.index.names = ['Continent', '% Renewable']
```
and this will satisfy the index naming requirement of Q-12