Added two functions: a better, faster way to calculate centroids and a crude algorithm for finding the poly of inaccessibility for each region in a shapefile.

The centroid function is computationally cheap. The pole of inaccessibility function, however, is very expensive. It works by fist creating a grid of n*n points. It checks every point to see if it is in the polygon, and if it is, finds the minimum distance to the border of the polygon. It's an algorithm that becomes exponentially more costly the larger n becomes. It's a crude algorithm in the sense the pole of inaccessibility problem is conventionally solved by forming a multiple n*n grids - each time an point is found, it creates another n*n grid to clarify the PoI even more. It's a feature I may add in the future.