Cleanup targets.py

[felker]

• [ ] Consider renaming TTDTarget to LogTTDTarget for clarity
• [ ] 7x threshold_range() class methods are never called anywhere. Vestige of old ROC calculation, e.g. in https://github.com/PPPLDeepLearning/plasma-python/blob/v0.9/plasma/utils/performance.py
• [ ] MaxHingeTarget.loss() (method unique to that class) is unused
• [ ] Base class Target is an invalid configuration option (predicts TTD directly, without the 0 mask before T_warning like in TTDLinearTarget?)

Related to #53, and the broader efforts of #42.

[felker]

Related: eb8f66484bec58ec3a1c13da4f503e5147ae8d3d