Rename `reactor-plc` to `fission-plc`

[MikaylaFischler]

Once fusion is added later, this is needed to differentiate. It should be done for the full release.

[dirkschumacher]

Thanks for the feedback. #33 and PR #36 are related to this interface idea.

I also like the ompr interface, but I would also like to have direct control over the model/ without any non-standard evaluation. One problem with e.g. add_variable(model, x[i, j, k], ...) is that it if used within a package you will get an R CMD check error, due to unassigned variable usage.

But yeah, I want to offer an api that is very similiar (or even fully compatible) to ompr but also have a more lower level interface using the R6 class. At least for now. Some steps have been made in PR #36 :)