Enable reading R0 data from ctapipe processor

[nietootein]

Reading from R0/R1 is currently missing and is needed for those projects that take waveforms as input.

[TjarkMiener]

R1 is already implemented in the subclass DLWaveformReader. I'm suggesting to implement a new subclass (e.g. DLTriggerReader) that reads from R0 data and do the trigger patch selection as it was done in the previous version of dl1dh. @jbuces I'd explore if it would be possible to inherit from DLWaveformReader, so you have all the config settings from it.

[jbuces]

Pdf file with a summary of the new output option of the DLRawTriggerReader subclass.

Running times of DLRawTriggerReader.pdf