[mobt-1157] Fix decision tree returning more than one threshold

[jdjonathandavies]

Description

Part of work on weather symbol decision tree in motb-1157

When adding decision tree nodes using screen temperature the decision tree would raise an exception. After debugging this issue I identified the cause being too loose a tolerance when selecting thresholds.

Solution

• Loosen the threshold tolerance from 0.01 to 0.001
• Add some comments around the steps creating the iris.Constraint used to extract the threshold
• Add an additional check on the extracted threshold cube to ensure that only a single threshold is extracted
• raise a ValueError with an informative message if there are more than 1 threshold extracted