Improve rerun.notebook.Viewer constructor

[jprochazk]

For a while now, the use_global_recording param of rr.notebook.Viewer didn't actually do as advertised. Setting it to False would make the Viewer constructor ignore (almost) all other parameters.

The original intent for that parameter was to allow creating a completely empty Viewer. The new behavior is:

• use_global_recording defaults to False if a url is provided.
• use_global_recording defaults to True otherwise.
• If no explicit recording is provided, and use_global_recording is True, then we use get_data_recording to pick up either the thread-local or global recording.

This means it is now possible to create a completely empty Viewer, and that is the default behavior given no params, and no global recording:

import rerun as rr

rr.notebook.Viewer()

If a url is provided, then by default we only show that url:

import rerun as rr

rr.notebook.Viewer(url="https://app.rerun.io/version/nightly/examples/arkit_scenes.rrd")

Using both a url and either an explicit/global recording is still possible:

import rerun as rr

rec = rr.RecordingStream("rerun_example_dual_recording")
rr.notebook.Viewer(
  url="https://app.rerun.io/version/nightly/examples/arkit_scenes.rrd",
  recording=rec,
)

# alternatively, call `add_recording` later:
rec = rr.RecordingStream("rerun_example_dual_recording")
viewer = rr.notebook.Viewer(
  url="https://app.rerun.io/version/nightly/examples/arkit_scenes.rrd",
)
viewer.add_recording(rec)
viewer.display()

# or just let it pick up the global recording:
rr.init("rerun_example_global_recording_notebook")
rr.notebook.Viewer(
  url="https://app.rerun.io/version/nightly/examples/arkit_scenes.rrd",
  use_global_recording=True,
)

If no url is provided, we pick up the global recording by default, but it can be turned off:

import rerun as rr

rr.init("rerun_example_global_recording_notebook")
rr.notebook.Viewer() # uses global recording

rr.init("rerun_example_global_recording_notebook")
rr.notebook.Viewer(use_global_recording=False) # will be empty

[Wumpf]

nice, way better! And thank you so much for updating the example notebook