Set default build type to RelWithDebInfo

Arndt, Danielrequested to merge
github/fork/masterleinad/default_build_type into master

Created by: masterleinad

Almost every time I configure ArborX from a clean directory, I realize a short time later that the default build type neither gives debug flags nor optimization flags and is thus pretty useless. In xSDK mode, the default is Debug so I choose that one here. I would also be happy to not allow an empty build type, though.

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