feat(qe): support pw.x AIMD output

dpdata/formats/qe/__init__.py

@@ -1,5 +1,5 @@
 from __future__ import annotations
 
-from . import scf, traj
+from . import pwmd, scf, traj
 
-__all__ = ["scf", "traj"]
+__all__ = ["pwmd", "scf", "traj"]

dpdata/formats/qe/pwmd.py

@@ -0,0 +1,14 @@
+"""Quantum ESPRESSO ``pw.x`` molecular-dynamics output reader.
+
+The implementation is shared with the ``qe/pw/scf`` reader because a PWscf
+output can contain either one labeled structure or a sequence of MD frames.
+"""
+
+from __future__ import annotations
+
+from .scf import get_frames
+
+
+def to_system_data(fname, begin=0, step=1):
+    """Read PWscf AIMD frames from an output file and its matching input."""
+    return get_frames(fname, begin=begin, step=step)