Add norm option to Riemannian trust-region and proximal-newton solvers

Add the option to choose different norm (H1-seminorm) for stopping criterion of these solvers.

requested review from @osander

assigned to @s7603593

Thanks Klaus. This is good, but it would be better still if the norm type wouldn't be saved as a string. Can you introduce an enumeration type like enum class ErrorNormType {infinity, ...} and use that to store the requested norm type?

added 1 commit

• 97d6e160 - Replace string for normType with enum class type

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merged

mentioned in commit 3d3f2abf