single eps: should we use euclidean distance as default?

[scheidan]

If a user provides multiple stats and uses type = :single, should we

1. throw an error, or
2. compute the (unweighted) euclidean distance of all stats?

Currently we do (2) but I think I'd prefer (1). There is not reason why we can assume that an unweighted distance is meaningful in general.

[scheidan]

I went for option 2.
Still need to be documented

[scheidan]

• no automatic distance computations
• find a way how to log individual components in the user code -> make example
• type = :single becomes a special case of type :multi, remove options. New default :hybrid

[scheidan]

done with 8ac7bf4