Offer robust implementation for scan averaging

[gdevenyi]

If we implement #7 we might as well make sure that we can use the same code at the averaging stage as well.

We can also generate an outlier map.

[gdevenyi]

Should have

• median
• trimmed mean (https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.trim_mean.html)
• huber estimator (https://www.statsmodels.org/stable/generated/statsmodels.robust.scale.Huber.html)

[gdevenyi]

Done with #49