imaging_depth missing from build_tidy_df output

[matchings]

A student at the Neuromatch course noticed that the imaging_depth is None in the output of the build_tiny_df function (or perhaps it was event_triggered_response). I believe this occurred after merging multiple experiments within a session together, so it may due to the way things were merged. In any case, to identify and solve the problem, the neuromatch tutorial notebooks should be run all the way through and each instance of an output with the imaging_depth column should be checked to ensure that it has a value:

https://github.com/NeuromatchAcademy/course-content/blob/main/projects/neurons/load_Allen_Visual_Behavior_from_SDK.ipynb

https://github.com/NeuromatchAcademy/course-content/blob/main/projects/neurons/load_Allen_Visual_Behavior_from_pre_processed_file.ipynb