Fix build failure related to ProductManifold and std::cout

The class ProductManifold has a bug that leads to a build failure when using std::cout from within the namespace Dune::GFE. This merge request adds a test for this, hopefully (in the future) a fix.

@amueller , this is your code. Could you have a look?

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• 2298ae45 - move friend function definition to inside of the ProductManifold class

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• 79799c33 - Fix build failure related to ProductManifold and std::cout

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Thank you!

mentioned in commit 8997acc4

merged

Somehow this fix is working. But I really didn't understand why it complained in the first place with an empty ProductManifold<>. Anyway, you're welcome.