Check Centroid Condition for AnnData and cells.json

[ilan-gold]

Right now, I believe the xy part of the cells.json (which is created from AnnData loaders and validated) needs to be part of the data because it is used for getting cell coordinates i.e the centroid needs to be included. We should thus throw an error on the AnnData loader if the polygons (i.e poly in the cells.json schema) are passed in but the centroids (xy in that lingo) are not:

https://github.com/vitessce/vitessce-python/blob/master/vitessce/wrappers.py#L365