Add references to the benchmarks used in OptimizationProblems.jl

[frapac]

It is not obvious to figure out which benchmarks are implemented in OptimizationProblems.jl. How about the following?

1. Add in the README.md a reference to all the benchmarks being used in OptimizationProblems (HS, CUTest, COPS).
2. Sort the files in proper subdirectories, as

   src/ 
   JuMP/
    COPS/
    HS/
    CUTest/

[tmigot]

Hi @frapac ! Very sorry for the late reply, I definitely like the idea of highlighting better the benchmarks that inspire OptimizationProblems. I think we can keep track of what is done in open and closed issues.

1. looks like a good idea, we could also have a markdown in the documentation if further details is needed.
2. I am a bit unsure about this, because problems may be found in several existing benchmark and we don't want to duplicate. Could we do something based on the OptimizationProblems.meta? Maybe adding a new column?

[dpo]

It’s not to direct because HS ⊂ CUTEst. Instead, we’ve been trying to indicate the source of each individual problem in the literature. Perhaps we could somehow add a column to meta to point to the “source” (a DOI would be great, but many references are old and don’t have one).

[dpo]

I think whatever comes out of #319 will solve this.