[feature] Add an abstraction layer to simplify calling routines from other codes

[drobnyjt]

Is your feature request related to a problem? Please describe.
Currently, issue #62 concerns adding binning of particle output for coupling to other codes. However, I'm concerned about adding some sort of binning/post-processing routine to RustBCA proper because of future maintenance costs and feature creep.

Proposed solution
However, a good solution would be to add an abstraction layer that includes the input file, so that a separate code can just use RustBCA as a library and call it and do the processing in memory straight out of single_ion_bca(). This would also have the benefit of simplifying main.rs and adding input.rs and potentially output.rs for code readability.

Alternative solution(s)
Python wrapper of Rust code would be simpler, but can streaming data from RustBCA be done succinctly?