optimize build

[bigerl]

The builds take too long for. This applies to tests and to using it elsewhere as a header only library.
Strategies to reduce the build time could involve:

• make it a static/dynamic library
• build non-templated classes with .cpp/.hpp
• use precompiled headers https://cmake.org/cmake/help/git-stage/command/target_precompile_headers.html
• use external template declarations (might be very tricky to keep track)
• declare typical template instantiations and compile them into a library
• use a build cache like ccache (if applicable)