Change Obs_poly to read shapefile inputs

[andrewzm]

Obs_poly could be changed to directly read shapefile inputs? This would supercede the current split file approach (trends and locations). The reason this would be good as currently, the way the function works, it forces you towards regular polygon shapes, which may not be the case with some of the newer datasets. Just think it may improve the end-user flexibility if it's possible to do.