QE: add metaInfo for cell mass in MD/relax simulations

meta_info/nomad_meta_info/quantum_espresso.nomadmetainfo.json

@@ -654,6 +654,15 @@
         "section_method"
       ]
     },
+    {
+      "description": "Mass of cell in MD/relax calculation",
+      "name": "x_qe_md_cell_mass",
+      "dtypeStr": "f",
+      "shape": [],
+      "superNames": [
+        "section_system"
+      ]
+    },
     {
       "description": "Finite E-field: direction",
       "name": "x_qe_berry_efield_direction",