feat: add DSL support

[ycexiao]

What problem does this PR address?

Add DSL(Domain Specific Langguage) support in diffpy.apps. It is migrated from PDF-bl/pdfbl.sequential#46, and details can be find in that PR.

What should the reviewer(s) do?

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[ycexiao]

@sbillinge, it's ready for review.

[sbillinge]

fantastic! @ycexiao do we need docs for this now though?

[ycexiao]

@sbillinge, Of course, we need give instructions about how to create the .dp-in file. I will create a issue and make a PR to it.

[ycexiao]

@sbillinge, this PR didn't create command entry point. My thought is:

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

Or as diffpy.apps refers to many applications, we can use something like:

diffpy-run(diffpy-exec) run <.dp-in>  

[sbillinge]

Without giving it to much thought, yes. Diffpy.apps should support multiple apps that could have different names and run different functionality in general