Geometric mean and median as reduction functions

This is a feature request for adding geometric mean and median to the list of reduction functions in datashader. For now, a work around for doing geometric mean is to use `ds.mean()` on log transformed data during aggregation, and then transform back.

Start by locating datashader's existing reduction functions and their tests, then compare how mean and other aggregations are defined. Implement support for geometric mean and median as reductions, and confirm both are covered by the relevant reduction tests, including the geometric-mean behavior described in the issue.

Written by the indexing model from the issue text.