Add triplet margin for distance functions in TripletEvaluator

[zivicmilos]

Problem:

Triplet margin exists in TripletLoss but does not exist in TripletEvaluator.

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Motivation:

When evaluating the model using TripletEvaluator during, for example, the training it would be a nice thing to have a piece of information not only if the negative is further away from the anchor than the positive, but also if it is further away from the anchor by some predefined margin. Because the triplet evaluator uses 4 different distance metrics, we would need an option for each one, so 4 different margins for each of them.

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Solution:

In the commit, I implemented exactly that, option for setting the margin for each distance function (or any subset of them). It is an optional argument to the class constructor, so it can be called perfectly the same as previously, with the difference that now you have more flexibility in setting how far away you want your negative and positive to be for evaluation.

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PS. I can also write some evaluator tests, both for TripletEvaluator with and without the margin.

PPS. I ran a make check, so the code follows coding guidelines.

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