This commit aims to enable the re-ordering of window operators in order to optimise
the sort and partition operators.
Example :
```
SELECT m1, m2,
SUM(m1) OVER(PARTITION BY m2) as sum1,
SUM(m2) OVER() as sum2
from numFoo
GROUP BY m1,m2
```
In order to compute this query, we can order the operators as to first compute the operators
corresponding to sum2 and then place the operators corresponding to sum1 which would
help us in reducing one sort operator if we order our operators by sum1 and then sum2.
27cfe12f4a