fromDict Should Handle Data Types with no URL

[ilan-gold]

vitessce-python/vitessce/config.py

Lines 713 to 721 in 80ba29b

	# Add each dataset from the incoming config.
	for d in config["datasets"]:
	new_dataset = vc.add_dataset(uid=d["uid"], name=d["name"])
	for f in d["files"]:
	new_file = new_dataset.add_file(
	url=f["url"],
	data_type=f["type"],
	file_type=f["fileType"]
	)

The above requires that the file object contains a url which is not necessarily true, for example, for raster data which have options not `url.

[ilan-gold]

Needed for (or at least a use case could be made for using this in): https://github.com/hubmapconsortium/portal-ui/tree/ilan-gold/initial_z_stack

[ngehlenborg]

Will likely be addressed by #115.