Re-introduce possibility of running ImageExtractor on waveforms with multiple gains

[maxnoe]

Please describe the use case that requires this feature.

Calibration events won't be gain selected, as we need to have calibration coefficients for all pixels for all gains.

Describe the solution you'd like

Make ImageExtractors work with non-gain-selected input again

Describe alternatives you've considered

Using a python loop over the gain channels with ManualGainSelector, but that could be much less efficient for ImageExtractors that could easily be vectorized over multiple gain channels.

[maxnoe]

We discussed this during today's developer call.

The conclusion was that we:

a) Want to reintroduce the feature that extractors can work on two gain channels and that should result in two images, one per gain channel

b) R1 and DL0 waveforms should always have the shape (n_channels, n_pixels, n_samples), even if n_channels is 1.

Together with the event type, pixel status and the camera readout, it is then clear whether gain selection has happened or the camera has just one gain.