Handling track-truth time offsets in Metrics

[rjgreen-dstl]

The metrics calculation needs to be able to handle instances where the ground truth and track objects have a time offset which occurs with real-world data.

It should be able to handle two cases:

• Ground truth interpolation (extrapolation?) to align with detection and track time stamps
• Acceptable tolerance level to linking ground truth with detection (e.g. +/- 0.25s, user defined) The approach should also handle rounding errors

[sdhiscocks]

Related to #364