Function returning references

That's something I had in mind for quite some time. Since Trixi's capabilities are expanding quite a lot, I think it would be really great to have some function, say `references`, that returns a list of papers you should cite when using some stuff. For example,
```julia
julia> references(Trixi)
@online{schlottkelakemper2020purely,
title={A purely hyperbolic discontinuous {G}alerkin approach for
self-gravitating gas dynamics},
author={Schlottke-Lakemper, Michael and Winters, Andrew R and
Ranocha, Hendrik and Gassner, Gregor J},
year={2020},
month={08},
eprint={2008.10593},
eprinttype={arXiv},
eprintclass={math.NA}
}
@misc{schlottkelakemper2020trixi,
title={{T}rixi.jl: A tree-based numerical simulation framework
for hyperbolic {PDE}s written in {J}ulia},
author={Schlottke-Lakemper, Michael and Gassner, Gregor J and
Ranocha, Hendrik and Winters, Andrew R},
year={2020},
month={08},
howpublished={\url{https://github.com/trixi-framework/Trixi.jl}},
doi={10.5281/zenodo.3996439}
}
```
Then, I would like to see something along the lines of
```julia
julia> trixi_include(some_elixir)
...

julia> references(sol)
[references to Trixi as above]
[references to SummationByPartsOperators.jl if you use FD methods]
[references for numerical fluxes you're using]
[references for split forms if you're using them]
[...whatever...]
[reference to the ODE solver]
[reference to DifferentialEquations.jl if you're using it]
[reference to the Julia paper]
```
What do you think about this?