Migrate to Zarr 3

Pre-work to #6

This MR has the following:

1. Migration to Zarr 3 API while removing the deprecated yet still supported Zarr 2 API usage.
2. Essential fixes for the main-branch resulting from the learnings out of the global-hackathon exploits (The non-essential ones will be fixed in a seperate branch).

apps/hiopy/worker.py

@@ -132,7 +132,7 @@ def main():
             t0 = (
                 np.datetime64(start_datetime())
                 - np.datetime64(var_group["time"].attrs["units"][len("seconds since ") :])
-            ) / np.timedelta64(1, "s")
+            ) / np.timedelta64(1, "s") + dt
             t0_idx = np.searchsorted(var_group["time"], t0)
             assert var_group["time"][t0_idx] == t0, f"start_datetime {t0} not found in time axis at index {t0_idx} which has value {var_group['time'][t0_idx]}"