[Feature] add support for multiple sources

[dannda]

Is your feature request related to a problem? Please describe.
hms-dbmi#259

Describe the solution you'd like
Option to provide multiple sources to visualize in the same view

[AlexSurtees]

Tested and seems to be working correctly. Do you have any example sources that I can use to look at an actual use case for this feature (currently I just used completely different images to test the feature).

If you're happy that this issue is finished @dannda, I'm happy to close it.