specify which step to combine in create_task_makefile

We may want to support another argument to create_task_makefile that allows you to specify which step you want each combiner to apply to. This would apply to the use case where, for example, step 2 creates a data.frame and step 3 creates a png, and combiner 1 builds a big data frame from all the step 2 data.frames while combiner 2 builds a big gif from all the step 3 pngs.

Start by locating the implementation of create_task_makefile and inspect how it represents steps and combiners. Define the expected argument behavior from the data.frame and png example, then verify that separate combiners can select their intended steps and produce the corresponding outputs.

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