draft implementation of crosscorrelogram function

[btel]

This is my implementation of cross-correlograms. It is not final yet - there are still some work to be done, but hopefully it will initiate some discussion on how to implement a general crosscorrelogram function.

I will appreciate your feedback, especially on:

• the API - what arguments and in what format the function should have,
• which algorithm and normalisation to use in the implementation.

[alperyeg]

@btel can you have a look here #63 and close this PR?

[apdavison]

Replaced by #63