PERF: added check for multi-geometries (#785)
PERF: added check for multi-geometries (#785)

This PR is related to Issue #698.

Added check in `_flatten_multi_geoms` to see if there are multi-geometries and whether the flattening is needed.
geopandas/geopandas,jdmcbr/geopandas,jorisvandenbossche/geopandas,ozak/geopandas,jorisvandenbossche/geopandas,geopandas/geopandas,jorisvandenbossche/geopandas,geopandas/geopandas,jdmcbr/geopandas,ozak/geopandas