Implement a basic command-line interface

This would useful for exploratory work, teaching and quick scripts. It should not provide too many options; e.g. it can only access composite functions rather than select implementation details. https://github.com/pace-neutrons/abinslib/issues/32

A useful parameter that doesn't add too much complexity would be to provide argument(s) for a group_by() call before plotting spectra. That should cover most use-cases especially if labels are implemented nicely. (https://github.com/pace-neutrons/abinslib/issues/35)

*Duplicate from original location https://github.com/pace-neutrons/abinslib/issues/34*

Start by reading the linked issues 32 and 35, then check the original issue 34 to understand where this duplicate's work belongs. Define the smallest command-line interface for exploratory use, including composite functions and optional group_by arguments before plotting spectra; done means those stated use cases work without exposing implementation details.

Written by the indexing model from the issue text.