Adding stub methods to LightCurve that child classes must implement.

[drewoldag]

After conversation with @AmandaWasserman, these two methods seem to be the only two that are required to be implemented by the subclasses of LightCurve.

[github-actions]

Before [04d3e6b]	After [b84746f]	Ratio	Benchmark (Parameter)
144±2ms	152±7ms	1.06	benchmarks.time_feature_creation
155±5ms	157±3ms	1.01	benchmarks.time_learn_loop('KNN', 'RandomSampling')
195M	195M	1	benchmarks.peakmem_learn_loop('KNN')
186M	186M	1	benchmarks.peakmem_learn_loop('RandomForest')
161±3ms	161±1ms	1	benchmarks.time_learn_loop('KNN', 'UncSampling')
2.61±0.01s	2.61±0.02s	1	benchmarks.time_learn_loop('RandomForest', 'RandomSampling')
2.67±0.02s	2.67±0.02s	1	benchmarks.time_learn_loop('RandomForest', 'UncSampling')

Click here to view all benchmarks.