MultiFileTileSource: pass dict on initialization

[banesullivan-kobold]

Instead of passing a file to MultiFileTileSource containing the YAML spec, could we implement a mechanism to pass a pre-built dict?

I find myself programmatically composing different images together and want to be able to easily create a tile source from that without writing out the YAML file.

This should be a straightforward change and I'm game to help if welcome

[manthey]

I think if you checked if the path was a dict and handled it adjacent to https://github.com/girder/large_image/blob/master/sources/multi/large_image_source_multi/__init__.py#L437-L444, validating it, it would work. It might only work if noCache=True is passed, since the caching may choke on a dictionary (but I'd try it first).