Improve the evaluate()-method

[1313e]

Rewrite/improve the evaluate()-method to make it very clear what part of the results belongs to what data_idx, as they are currently returned in the order PRISM handles them (which has no logic for the user at all).

• Split results up into the individual emulator iterations at which the sample was last evaluated;
• Every result must show to which data point (data_idx) it belongs to;
• Allow for a subset of data_idx to be given?