Implement a ManifoldEuclideanGradientObjective

[kellertuer]

I am not yet sure how to provide this easily in the high-end interfaces, but with the new structure in Manopt.jl 0.4 it is easy to provide a ManifoldEuclideanGradientObjective, that is a cost function $f$ together with its Euclidean gradient (usually called egrad in Manopt), since every gradient call would just be encapsulated in a projection onto the tangent space and a change_representer call.

The main task here is a nice way to handle this in the high-level interfaces. One way could be to extend the evaluation types as follows:

• Introduce AbstractAllocatingEvaluation and AbstractInplaceEvaluation as intermediate types that the current ones inherit from
• Introduce further subtypes AllocatingEuclideanEvaluation and InplaceEuclideanEvaluation

this makes it easy to specify that gradients (and later Hessians) are defined in the Embedding/Euclidean sense and the corresponding Objectives should be taken. One could maybe even just adapt the ManifoldGradientObjective in this way without introducing a new objective.

[kellertuer]

Oh, this one might require that change_representer is moved to ManifoldsBase.jl