Hello I'm having troubles dealing with Pandas. I'm trying to sum duplicated rows on a multiindex Dataframe.
I tryed with `df.groupby(level=[0,1]).sum()` , also with `df.stack().reset_index().groupby(['year', 'product']).sum()` and some others, but I cannot get it to work. 
I'd also like to add every unique product for each given year and give them a 0 value if they weren't listed.

Example: dataframe with multi-index and 3 different products (A,B,C):

```
                  volume1    volume2
year   product
2010   A          10         12
       A          7          3
       B          7          7
2011   A          10         10
       B          7          6
       C          5          5

```

Expected output : if there are duplicated products for a given year then we sum them. 
If one of the products isnt listed for a year, we create a new row full of 0.

```
                  volume1     volume2
year   product
2010   A          17          15
       B          7           7
       C          0           0
2011   A          10          10
       B          7           6
       C          5           5

```

Any idea ? Thanks