ophys.response_analysis.cell_metrics throws a PerformanceWarning

```
df = cell_metrics.load_metrics_table_for_experiments('all_experiments','traces','full_session','full_session', use_events=True, filter_events=False)
```

> your performance may suffer as PyTables will pickle object types that it cannot map directly to c-types

Can we either fix this, or suppress the warning?