Add support for mode

[sehrgut]

It would be useful to support mode, not just mean and median.

[lulunac27a]

Mode is the number that is most often of the set of numbers
For example:
(2, 2, 2, 3, 3, 5, 7, 8, 10, 10, 12, 15) is 2 because number 2 happens most often