chore: entry point for running .dp-in file #6

Problem

The entry point for running .dp-in file has still not been settled down.

As mentioned in #3 (comment), we can actually use different entry points for different applications. And for running .dp-in file, we can use

diffpy-run --file <.dp-in>

But stick with one entry point is easier for user to manage. As more applications being implemented in the future, user might not want to get a bunch of unfamiliar commands from one single package. In this case, the entry point can be

diffpy.app runfile <.dp-in>

Here the s is removed from diffpy.apps because only one app, runfile, is invoked.

Proposed solution