Factor Out Common Distance API to Leverage eval_op, ... etc.

[bhgomes]

I think it would be beneficial to factor out the eval_op, eval_end, eval_reduce API so that others who want to define custom metrics (like #95) can extend the already optimized evaluate that is built off of them. Not sure how this would interact with UnionMetrics.

[dkarrasch]

@bhgomes Could it be that this is resolved by #163? Depending on what you want to define, PreMetric, SemiMetric or Metric, you can subtype your custom metric to Union[Pre/Semi]Metric, overload those functions that are not covered by the generic fallbacks, and otherwise let the package do its job. See #151 (comment) for an example.

[dkarrasch]

Closed by #163.