Standard Deviation

This pull request implements a new variable - standard deviation of a grid cell is returned, optionally, if `--std` is set. Currently only implemented in-core, since the current implementation doesn't parallelize well.

The standard deviation is calculated using a numerically stable Welford algorithm provided here: https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Online_algorithm

Note that the standard deviation implementation provided here is prone to the same bugs that the mean, min, and max are all exhibiting at this point in time and described initially and partially in  https://github.com/CRREL/points2grid/issues/5, https://github.com/CRREL/points2grid/issues/6, and https://github.com/CRREL/points2grid/issues/7