FEATURE: Cross-correlation process

Summary

Cross-correlation process (autocorrelation being cross-correlation of an array with itself).

Current behaviour/setbacks

There isn't one.

Desired behaviour/advantages

Process that takes one or two dataframes and computes cross-correlation, along with interquartile ranges. If one dataframe, perform autocorrelation.

Implementation sketch

See files written by Peter attached, particularly autocrosscorr() in time_series_analysis.py. I only just need to slightly change the I/O to fit other post-processes in core.multisignal, where this cross-correlation process should live.