Build fail on MacOS: test_routines fails to link as it is empty

Building elsa (master) on a MacOS with AppleClang fails, as ar fails to link libelsa_test_routines since there are no files to link.

As everything in there is a template, this is quite annoying. We could maybe put the implementations of generatedRandom* into a cpp file with explicit template instantiations for the usual types (as we do in the other elsa modules).

Opinions?