Implement aggregation operations

[sharadgaur]

Following functions need to be implemented:

• group (all values in the resulting vector are 1)~
• stddev (calculate population standard deviation over dimensions)
• stdvar (calculate population standard variance over dimensions)
• count (count number of elements in the vector)~
• count_values (count number of elements with the same value)
• bottomk (smallest k elements by sample value)
• topk (largest k elements by sample value)
• quantile

[yeya24]

One note here, we have group and count already.
It is easier to start with stddev and stdvar as they takes a range vector only, while count_values, bottomk and topk need to take additional parameters so probably we can wait for #23

[alanprot]

Hi,

I did not see this issue before so i opened a new one for bottomK and topK. Those aggregations seems to need some additional work so i think it should be fine to track those there?