Address header-only repositories

[ednolan]

Currently beman.exemplar is morally header-only but still builds a static library. We need a separate exemplar repository for header-only libraries, or make beman.exemplar header-only and have a separate exemplar for static libraries. We need to decide on what our CMake will look like for header-only Beman libraries.

[ednolan]

Created a WIP commit to address this: bemanproject/exemplar#262

Remaining TODOs on that commit:

• Update cookiecutter so that users can optionally configure the repository to create a src directory/static library/etc
• Update CI so it doesn't run a shared library build on the header only configuration
• Update the README to reflect an accurate install tree
• Get feedback from @nickelpro about where to put the beman.examplar-config.cmake.in file (see https://discourse.bemanproject.org/t/converting-exemplar-to-a-cmake-interface-type-library/527/2?u=ednolan)
• Add a CI test that generates the static-library-based version of exemplar and runs a smoke test to ensure that it builds