Move podio_PYTHON_DIR to the top level CMakeLists

[jmcarcell]

BEGINRELEASENOTES

• Move podio_PYTHON_DIR to the top level CMakeLists so that it is set even when BUILD_TESTING is off

ENDRELEASENOTES

This is useful when building other packages at the same time and having BUILD_TESTING=OFF. Alternatively, to reduce the number of lines in the top level CMakeLists the if(BUILD_TESTING) could be moved inside the tests/CMakeLists.txt and this variable could be always be set there

[tmadlener]

Shouldn't dependent packages get this via the find_package, where it is set? This is mainly a curiosity, I don't see any reason to not merge this.

[jmcarcell]

Yes if you build podio, install it and then build something else. No if you use add_directory or FetchContent to build podio together with some other packages. In general everything that is not being hardcoded like this path works fine with this approach

[tmadlener]

Thanks for the clarification.