3dmol format for visualization in jupyter (#357)

Example:
https://nbviewer.org/gist/njzjz/14a638d1b5c85c1b2a4882418c34dd9d


![image](https://user-images.githubusercontent.com/9496702/192133036-cfacfab4-ab9c-482d-af25-412116008d9d.png)

Author: njzjz

dpdata/plugins/3dmol.py

@@ -0,0 +1,44 @@
+from typing import Tuple
+import numpy as np
+
+from dpdata.format import Format
+from dpdata.xyz.xyz import coord_to_xyz
+
+
+@Format.register("3dmol")
+class Py3DMolFormat(Format):
+    """3DMol format.
+
+    To use this format,  py3Dmol should be installed in advance.
+    """
+    def to_system(self,
+                  data: dict,
+                  f_idx: int = 0,
+                  size: Tuple[int] = (300,300),
+                  style: dict = {"stick":{}, "sphere":{"radius":0.4}},
+                  **kwargs):
+        """Show 3D structure of a frame in jupyter.
+
+        Parameters
+        ----------
+        data : dict
+            system data
+        f_idx : int
+            frame index to show
+        size : tuple[int]
+            (width, height) of the widget
+        style : dict
+            style of 3DMol. Read 3DMol documentation for details.
+
+        Examples
+        --------
+        >>> system.to_3dmol()
+        """
+        import py3Dmol
+        types = np.array(data['atom_names'])[data['atom_types']]
+        xyz = coord_to_xyz(data['coords'][f_idx], types)
+        viewer = py3Dmol.view(width=size[0], height=size[1])
+        viewer.addModel(xyz, 'xyz')
+        viewer.setStyle(style.copy())
+        viewer.zoomTo()
+        return viewer