Can't add `tmp_data_to_save` to old merged analyzer

[chrishalcrow]

As of v0.104.0 template_metrics saves some tmp_data_to_save: the peak_data and main_channel_templates alongside the metrics as data in the extension.

This creates a backward compatibility problem. When you load a v0.103.2 analyzer using v0.104.0, this has no peak_data. So when you try to e.g. merge two units, the analyzer cannot update the peak_data (since it has no peak data). Actual bug is here

spikeinterface/src/spikeinterface/core/analyzer_extension_core.py

Line 1416 in 962087b

self.sorting_analyzer, new_sorting_analyzer, self.data[k], new_tmp_data[k], new_unit_ids

self.data has no key "peak_data".

Wanted to raise an issue cause I think this is a general problem and there are a few solutions:

1. Use get instead of indexing, and don't update data self.data didn't contain the tmp_data to being with.
2. Use _handle_backward_compatibility_on_load to add a suitable "empty" peak_data.

Number 1. feels easier but I actually quite like 2, because it ensures that no matter what you load with v0.104.0, you get the same data structure. Thoughts?

[alejoe91]

Yeah I'd go with option 2!