TST: Add tests for centroid and convex_hull (#568)
TST: Add tests for centroid and convex_hull (#568)

Basic tests of methods that were not previously covered.
jdmcbr/geopandas,jdmcbr/geopandas,geopandas/geopandas,jorisvandenbossche/geopandas,geopandas/geopandas,jorisvandenbossche/geopandas,jorisvandenbossche/geopandas,ozak/geopandas,ozak/geopandas,geopandas/geopandas