Add rf_agg_local_median aggregate function

[emc5ud]

Similar usage as rf_agg_local_mean

[metasim]

@emc5ud We need to identify a "streaming median algorithm" or "rolling median algorithm" to implement. . Some cursory research resources:

• https://stackoverflow.com/questions/1309263/rolling-median-algorithm-in-c
• https://stackoverflow.com/questions/31432843/how-to-find-median-and-quantiles-using-spark
• http://infolab.stanford.edu/~datar/courses/cs361a/papers/quantiles.pdf

We temporarily, we might be able to just explode the tile and feed it into the built-in scalar approxQuantiles implementation.

[vpipkt]

This should be closed with #429