Remove pure_to_mixed in favor of vector_to_density_matrix

[vprusso]

In favor of both pure_to_mixed and vector_to_density_matrix being functionally equivalent, it may make sense to do the following:

1. Remove the pure_to_mixed function (along with the associated tests)
2. Replace any usages of pure_to_mixed with vector_to_density_matrix.
3. Rename vector_to_density_matrix to just to_density_matrix as the input does not necessarily need to be a vector (indeed, it can even be a density matrix) so it may be misleading to prefix the name with vector (and hence should drop).

[mhaoda]

I'm a quite new contributor to opensource. I want to try a simple issue at the beginning, can I work on this?

[purva-thakre]

Hi @mhaoda, I think it might be better to pick a different issue. Someone else (@Shivansh20128) is interested in fixing this issue as they pointed out the the stuff in the description in the toqito Discord channel.

Good to know you are a new contributor! @vprusso or I could walk you through the steps to resolve a different issue.

[mhaoda]

Thanks so much @purva-thakre, I'm totally willing to look at a different issue, could you please provide more information about it?

[vprusso]

Appreciate your enthusiasm, @mhaoda !

Depending on @Shivansh20128 's inclinations, we can see if they are planning on tackling this. Alternatively, any of the other issues on the board that have the "good-first-issue" tag are typically low barrier-to-entry tasks that are good entry points to the project. I intend on continuing to add more issues with this tag as well, so do stay tuned.

If you like, I can tag you on other issues that I put up that might be good candidates for you if you are interested. Let me know, as I don't want to spam you needlessly! :)

[Shivansh20128]

Hi! Yes, I will work on this issue after finishing the previous one. I am very close to completing it.
Thank You