Create entry points for preprocess, infer, and postprocess

[dchaley]

Thus far we've been running DeepCell via the benchmark.py script which does all three steps in one.

We need separate entry points for each step. This means doing some surgery on the DeepCell library.

This task involves:

• determining in/out data at each step
• creating a way to call a specific step (simple: entry points or flags in the same container; fancy: different containers to optimize when not using GPU)

We'll set up the workflow in another step

[dchaley]

PR deepcell-imaging#233 performed a prerequisite refactor: pull instance methods into independent functions.

Now we need to set up actual entry points to call these. This will require architecting the input/output interface of these steps. 🤔

[dchaley]

See #234 for entrypoint interfaces.