Support multiple tensors per tensor page

[ShadenSmith]

Some data sources produce multiple tensors, such as:

• Chicago crimes
• Reddit (lots of options)
• Delicious
• Matrix multiplication

However, our tensors page only supports one tensor per data source. I propose we instead support a list of tensor datasets and to list them all on the collections page. This considers the files in the _tensors/ directory to be more like data sources which emit one of more tensors.

This will require some infrastructure work.

[fruitfly1026]

Yes, agree. Tensors generated from the same data source are quite interesting. We might need some different sub-names like "xx-[subname]" to distinguish them. I also can help with it. We can discuss more.