Improve Example Code Readability

Maybe this is because I'm coming from more of a software engineering background than an operations research background, but I find the [code examples](https://python-mip.readthedocs.io/en/latest/examples.html) in the documentation to be very dense and hard to read.

I like the examples, there are some interesting problems there and the code samples do show you how to use the `mip` package to get solutions to them.

However, if you put yourself in the shoes of someone who is evaluating `mip` and trying to imagine how it could be integrated into an actual production system, the code examples fall pretty short.

I figure this is a pretty low-priority issue, but I thought I should at least bring up the topic and provide an example of how the code samples could be improved.

I've re-written the [Plant Location with Non-Linear Costs](https://python-mip.readthedocs.io/en/latest/examples.html#exsos) example as a way of showing what could be possible. If you're interested, please compare my [re-written version](https://gist.github.com/mwerezak/8f2846a4fcace8825291a3632ffd97fc) with the original one. I believe my version is much more readable and would give a reader who is unfamiliar with `mip` a better sense of how it could be integrated into a larger system.