Auto correlograms

[yger]

This PR adds a new extension: auto_correlograms. This is a gift for @oliche

This is separated from correlograms because the internal data structure is simpler. Note that also as soon as #4305 is accepted, the fast_mode option of numba should be propagated also to this extension.

EDIT by Sam:
This new (faster) extension could be used both in the GUI but also for some metrics when correlograms extension is no here.

[oliche]

So I would just have to use method='auto' and I'll get the autocorrelograms ! Thanks @yger

[alejoe91]

Minor changes :)