Add example of collecting traces across planes for multiscope sessions

[matchings]

Since the SDK is at the imaging plane level, you have to load multiple planes to aggregate all the neurons recorded in a given session. Depending on the analysis, you may want to align them in time (since each mesoscope plane is acquired sequentially). We should have a simple function or code block that shows how to do this.

[saskiad]

Multiple planes are now all in the same NWB file, so you don't need to load multiple assets, but I do think that an example of how to work with data across simultaneous planes (that are sampled at different time points, etc) would be valuable to add.