Add CMake options for disabling docs, examples and tests

[tambry]

I'm trying to package this as a Debian package for use by team. Downloading things from the internet during a build is a big no-no for packaging (you ought to get them the necessary things from the repository). Since making the code work without Conan is more than I'm willing to bite and I only plan to package for internal use this simply adds options to turn off building docs, examples and tests. These are set to ON by default if mp-units is the top-level project, i.e. a normal dev setup.

[mpusz]

The build without docs, examples, and tests is just a build of the library, right? If this is true than https://mpusz.github.io/units/usage.html#cmake-conan should be enough. If you do not want to use Conan than you can work with src/CMakeLists.txt directly as this is meant to work independently and just provide a library to the end user (without changing his/her dev environment i.e. compilation warnings flags).

A top-level CMake entry point is meant for library development and this is why it should always build everything with really demanding compilation flags.

[tambry]

Using src/CMakeLists.txt seems like a fine option.