Update stereonet_math.py

[JNDanielson]

Currently, the find_fisher_stats, fisher_stats, and mean_vector functions don't calculate Fisher k (kappa) or the mean vector of a group of poles correctly where the group of poles includes orientations which cross a dip=90° (e.g., subvertical joints with steeply north and south dipping orientations will have a flat mean orientation instead of a subvertical one). This is because the mean vector calc assumes that orientations have directions, which is not true for most data plotted on stereonets (e.g. rock joints). See Issue joferkington#39 for an example of undesired results.

This update uses the scatter matrix approach from Directional Statistics (Jupp and Mardia, 2000, pp.165) to estimate mean orientation and dispersion. This fixes the aforementioned issue.