Conan package

We are currently installing `units` as a CMake external project. While this works, it requires some manual maintenance on our end (and on anyone else's who uses the library in this fashion). This would be alleviated if there were a Conan package of `units`.

I created a simple test recipe and managed to make it work locally. But it would of course be best to publish a package. We could do so on [conancenter](https://conan.io/center/) directly or on our own, public server as ESS.

I would like to get your input. Did you consider using conan or a different package manager at any point?

Start with the existing CMake external-project setup described in the issue and the local Conan test recipe. Determine whether a Conan package should target Conan Center or a public ESS server, then clarify the publication and maintenance requirements. Done means an agreed package-management path and a published, usable package.

Written by the indexing model from the issue text.