Add geometric mean to map marker plugins

[dmgaldi]

Currently, only median and arithmetic mean are supported. The challenge here will be avoiding overflow when multiplying numbers. This can be solved by doing arithmetic mean in the log-space, but this costs us efficiency. Another idea is to multiply until we hit a sufficiently large number and then do a log (e.g. https://stackoverflow.com/questions/19980319/efficient-way-to-compute-geometric-mean-of-many-numbers)