CameraReadout.from_file() should fail if no camerareadout exists

[kosack]

This caused a lot of confusion, and means most of the tests are not correct (since they fail to read a correct CameraReadout and fall back to a generic one that doesn't look at all like a Cherenkov pulse):

From readout.py:

        except FileNotFoundError:
            # TODO: remove case when files have been generated
            logger.warning(
                f"Resorting to default CameraReadout,"
                f" File does not exist: ({tabname})"
            )
            reference_pulse_shape = np.array([norm.pdf(np.arange(96), 48, 6)])
            return cls(
                camera_name=camera_name,
                sampling_rate=u.Quantity(1, u.GHz),
                reference_pulse_shape=reference_pulse_shape,
                reference_pulse_sample_width=u.Quantity(1, u.ns),
            )

So if you create a camera like:

camera = CameraDescription.from_name("LST")

You get a crazy reference pulse shape that doesn't reflect reality (it's 10x too wide and not properly normalized), and this is used in tests internally. I think the correct behavior would be to fail with an error, and we should add correct *.camerareadout.fits.gz files in ctapipe-extra so that doesn't happen.

[kosack]

Of course, we don't have camera_readout defs for some of the test cameras (HESS, Whipple-X, etc). So perhaps instead of simply failing, it should just not create the CameraReadout (leave it as None), that way tests that do not need timing will still work, but those that do not should fail on cameras with no timing info

[kosack]

As a quick fix, we should just add *.camreadout.fits.gz files to ctapipe-extra. I opened #1451 to facilitate that.