Negative lags for cross-correlation process

Summary

Option to choose between showing only positive lags and showing both negative and positive lags from the cross-correlation process

Current behaviour/setbacks

Only shows positive lags. Only useful for autocorrelation, not for cross-correlation.

Desired behaviour/advantages

Option to compute and write negative lags to output DataFrame. Showing negative lags will be useful for cross-correlation because it will allows us to decide whether signal A is behind signal B or vice versa.

Implementation sketch

• Add boolean parameter for the option (positive only or negative & positive)
• Computation: see https://git.ecdf.ed.ac.uk/swain-lab/wela/-/blob/master/autocrosscorr.py#L84 to end of function