improve treatment of equal-metric appraisals

[Aurashk]

• Right now there is a hardcoded approach to prioritising appraisals with equally-good metrics in AppraisalOutput::compare_with_equal_metrics. We may want to make this configurable

• If we still can't single out a single-best appraisal we always pick the first element in the container in select_from_assets_with_equal_metric. We may also want to make this configurable

• It might make more sense to remove the call to AppraisalOutput::compare_with_equal_metrics in AppraisalOutput::compare_metric and independently sort by the equal-metric criteria in select_from_assets_with_equal_metric instead of relying on the initial sort with compare_metric.

• Since the rules are getting more complex might be nice to have add testing for this functionality with an array of appraisals as a fixture which checks the asset we expect is chosen