Remove/clean unused types and classes

[mdabros]

Currently there are a few which contains functionality that is rarely used, or that doesn't quite fit into the current direction of the package. This includes:

• SharpLearning.Containers.Arithmetic: MatrixF64 is mostly used as a container, and more efficient matrix arithmetic can found in other libraries, like mathnet.numerics.
• SharpLearning.Containers.ObservationTargetSet: This can be replaced by using a value tuple instead.
• SharpLearning.Containers.ArrayExtensions: Several methods are unused.
• SharpLearning.CrossValidation.ContinuousMungeAugmentator:
• SharpLearning.CrossValidation.NominalMungeAugmentator: