Update contouring.py

[j-ahn]

Added fisher distribution to density_grid function

This is to replicate the Fisher density contours in Dips (Rocscience) https://www.rocscience.com/assets/resources/learning/papers/Dips-An-Interactive-and-Graphical-Approach-to-the-Analysis-of-Orientation-Based-Data.pdf

Comparison with dips