Unify the Euclidean metric types

[sethaxen]

We need 4 things from a Euclidean metric:

• a size
• left-multiplication by the metric M⁻¹
• a quadratic form dot(v, M⁻¹, v)
• left-division by W, such that W'*W == inv(M⁻¹)

Current this is handled by 3 different types:

• UnitEuclideanMetric
• DiagonalEuclideanMetric
• DenseEuclideanMetric

However, a user may want to initialize with a diagonal metric adapt a dense metric or vice versa. Assuming we only support metrics for which an AbstractMatrix representation is available, it seems like it would be preferable to instead have a single EuclideanMetric type that stores the matrices M⁻¹ and W (UniformScaling for UnitEuclideanMetric) and then have the convenient aliases:

const ScalarEuclideanMetric = EuclideanMetric{<:UniformScaling}
const DiagonalEuclideanMetric = EuclideanMetric{<:Diagonal}

for dispatch.

This would also eliminate the need for a custom metric type whenever a user wants to use a different matrix type for the metric for which a more efficient W is available then the Cholesky factor, see e.g. #277, #282