process metrics support HDF5 peri-event input with per-pixel metric maps

[celefthe]

`process metrics` (#152) reads the long-format CSV written by `process peri-event` only. The HDF5 output (`/traces` of shape `(n_trials, n_samples, H, W)`) is not supported.

should also accept the HDF5 file and compute the same per-trial metrics (onset, amplitude, AUC, decay) and session summaries per pixel, writing per-pixel metric maps to `_metrics.h5`.

[celefthe]

Per-pixel metric maps should also include the reliability metrics from #173.