Attribute extrema computation without child computation

[lefevrej]

Hi Benjamin,

I propose a modified version of attribute_extrema which does not require the children computation. Apart from the memory impact, I have done a benchmark on a graph with about 18 million edges and obtained a speedup of 20.

As I directly modified the existing function, I didn't have to modify the documentation or the associated unit test.

Thank you for your time and consideration.

Josselin

[PerretB]

Great contribution, thank you! Can you make the slight modification indicated above and it will be ready for merge

[PerretB]

Thank you!