DateMetricResult attributes name

[diegoamc]

The attribute metric_result on DateMetricResult should be named tree_metric_result. See here.

This is a leftover from the HotspotMetricResult introduction.

Create a new attribute called tree_metric_result and print a deprecation message.

Upon fixing this issue, a new one has to be created on Prezento to handle the deprecation. Only after Prezento is running with the new attribute the old one can be removed.