Create job that processes multiple inputs

Currently, we process one image at a time in our task specification. In particular the input is a single file url.

Instead, we need to run on a collection of images. In Batch, each task has the same arguments, and the runtime receives $BATCH_TASK_INDEX in the environment. We need to use this task index to determine which file to actually run on.

This means we need to provide the task definition with a list of files. Probably as easy as passing a URI to a list of files, and indexing out our file.

So, each task would do this:

fetch the list of files
select out my file by index
then do what we already do

dchaley / deepcell-imaging

Create job that processes multiple inputs #292