Request to implement Convolve and Correlate equivalent to NumPy versions.

[jjelovich-siemens]

See: https://numpy.org/doc/1.18/reference/generated/numpy.convolve.html
and
https://numpy.org/doc/1.18/reference/generated/numpy.correlate.html?highlight=correlate#numpy.correlate

[spectre-ns]

I have an implementation of convolution that is in review. I think correlation is a special case of convolution where zero padding is added to make it so you can convolve until there is no overlap remaining. If I remember correctly there is biased and unbiased where the biased version is normalized to the number of overlapping signal points. If someone wants to build on xt::convolve that would be great!

@JohanMabille can we implement correlate in a scipy port instead of xtensor because it looks like the python people have duplicated effort on correlate where they defer to scipy for long correlations anyways. Thoughts?