Improve Test style and coverage -- part 2

[mateuszbaran]

These are the points still remaining after PR #319 :

• Can we do a test for https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/groups/metric.jl#L216
• How can we best test the Powerdistribution? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/PowerManifold.jl#L191
• Can we test ProductFVectorDistribution? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/ProductManifold.jl#799
• Can we test ProductManifolds _rand? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/ProductManifold.jl#861
• Which errors do appear in Rotations? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/Rotations.jl#L438
• Can we make up a sopecial case for this situation in Rotations log!? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/Rotations.jl#L504
• Do we have a tangent space fibre bundle test do add this line? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/VectorBundle.jl#L617
• Similarly a tangent space type fiber should cover the norm, too? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/VectorBundle.jl#L736
• similarly the generic project https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/manifolds/VectorBundle.jl#L820
• Distributions are missing a non-implemented check https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/distributions.jl#L62
• We could add differential! and gradient! tests? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/riemannian_diff.jl#L27
• from the utils: Is eigen_safe for a StaticMatrix still needed or are they now part of `StaticArrays in some form? https://codecov.io/gh/JuliaManifolds/Manifolds.jl/src/master/src/utils.jl#L54

I've removed metric stuff from this list because we need to solve #6 first anyway.