Use __call__ where apropriate

[dneise]

We have some components in ctapipe, which are in principle callables and offer only one real interface, which is to call them:

Possible examples are:

• CameraCalibrator.calibrate()
• CameraDL1Calibrator.calibrate()
• CameraDL0Reducer.reduce()
• ChargeExtractor.extract_charge()
• DataVolumeReducer.reduce_waveforms()
• WaveformCleaner.apply()
• IntensityFitter.fit()
• Reconstructor.predict()

In #1022 Max thankfully starting tidying this up. This issue is just a reminder for us to tidy the rest up as well.