pyimagej - Add spatial statistics to convex_hull notebook

[carshadi]

• add function to compute geometric centroid of the terminal node convex hull and measure average distance of each tip to it. This provides insight into the central tendency of the endpoints.
• add function to compute the co-variance matrix of the end points and its trace, which quantifies the degree to which points in a point set are separated from each other.
• make printing statistics optional
• reformat some lines and update comments

Förstner W., Moonen B. (2003) A Metric for Covariance Matrices. In: Grafarend E.W., Krumm F.W., Schwarze V.S. (eds) Geodesy-The Challenge of the 3rd Millennium. Springer, Berlin, Heidelberg

“1.5 - Additional Measures of Dispersion.” Comparing Two Quantitative Variables | STAT 800, The Pennsylvania State University, newonlinecourses.science.psu.edu/stat505/node/21/.

[carshadi]

going to perform clustering prior to spatial stats, since it should give a more meaningful result