Add metric MultiManager and MetricPlotter

[rcgorman-dstl]

• Have replaced SimpleManager metric manager with new MultiManager class so this will be a breaking change.
• MultiManager can take multiple metric generators of the same type and multiple sets of the same type to generate metrics.
• All metrics that take in track and truth can now also take in 2 tracks.
• Additional functionality added to MultiManager through new methods to help easier extraction of metrics and insights.
• New MetricPlotter class created to make plotting metrics easier.

Update:

• Have added SimpleManager metric manager back in to avert breaking change. It should operate in the same way as before.

[sdhiscocks]

The breaking change makes this difficult to merge at this point. However, it may be possible to make it backwards compatible fairly easy:

• Add back in the SimpleManager, inheriting from MultiManager, with "tracks", "groundtruth_paths" and "detections" as default keys in states_set.
• Have property on SimpleManager for tracks, groundtruth_paths and detections that returns values from those dictionary keys mentioned above.
• Change metrics so default for tracks_key and truths_key are "tracks" and "groundtruth_paths".
• Also provide sensible default generator_name for each metric.

I've not tried this, but something like this should work or at least be a good start.