Turn examples into crates

[stefan-k]

Currently users are occasionally confused about how to properly add argmin and argmin-math to their dependencies. While it is explained in the book, it is sometimes not the first thing they find. Instead they occasionally start from the examples and use internal features (such as _nalgebral) to make it work.
By turning examples into their own crates, each example gets their own Cargo.toml and as such helps users to get started.

Related issues and PRs: #306, #270, #268