be deliberate about library install

[laroque]

See pybind/cmake_example#11 (comment) ; this seem to be dealing with precisely the questions we've had about getting binding helper functions into source files and also for dealing with where the libs get installed.

[laroque]

There seem to be fundamental issues with this, so for now we've refactored back to having a single source file in the binding and everything else is in a header. We'll leave it like this for now and try to refactor if/when there is support for a cleaner pattern.