Clean up getpolygon

the getpolygon functions are still optimized for interfacing with PyPlot. They return polygon data in a matrix instead of a vector of points, and because of python the matrix is also the wrong way round for Julia.

somebody should fix this (and adapt the meshing functions, the obstacle bin interactions, and SpatialCoarseGraining)

also, currently getpolygon does not work for 2D versions of 3D-enabled models as it is hardcoded to expect a field cell.φ to exist