make "coords" optional and infer natoms from "types" instead

doc/src/molecule.rst

@@ -814,7 +814,7 @@ for the "native" molecule file format.
      - overrides the auto-computed moments of inertia
    * - coords
      - a data block
-     - yes
+     - no
      - contains atom positions with the format "atom-id", "x", "y", "z" (same as Coords)
    * - types
      - a data block

src/molecule.cpp

@@ -280,8 +280,8 @@ void Molecule::from_json(const std::string &molid, const json &moldata)
     error->all(FLERR, Error::NOLASTLINE, "Found {} instead of {} data entries for '{}'", sizevar, \
                sizecheck, #field);
 
-  JSON_INIT_FIELD(coords, natoms, xflag, true, 0);
-  JSON_INIT_FIELD(types, dummyvar, typeflag, true, natoms);
+  JSON_INIT_FIELD(types, natoms, typeflag, true, 0);
+  JSON_INIT_FIELD(coords, dummyvar, xflag, false, natoms);
   JSON_INIT_FIELD(molecules, dummyvar, moleculeflag, false, natoms);
   JSON_INIT_FIELD(fragments, nfragments, fragmentflag, false, 0);
   JSON_INIT_FIELD(charges, dummyvar, qflag, false, natoms);

tools/json/molecule-schema.json

@@ -302,5 +302,5 @@
             "required": ["format", "data"]
         }
     },
-    "required": ["application", "format", "revision", "coords", "types"]
+    "required": ["application", "format", "revision", "types"]
 }