Added example in metatensor showing usage

src/metatensor/metatensor.cpp

@@ -34,7 +34,23 @@ directory defines a custom machine learning CV that can be used with PLUMED.
 
 \par Examples
 
-TODO
+The following input shows how you can call metatensor and evaluate the model that is described in the file soap_cv.pt from PLUMED.
+To evluate this model plumed is required to use code that is included in the directory extensions which has been specified using the EXTENSIONS_DIRECTORY flag.
+Numbered SPECIES labels are used to indicate the list of indices that belong to each atomic species in the model.  The 
+SPECIES_TO_TYPE keyword then provies information on the atom type for each species.  The first number here is the atomic number of the atoms
+that have been speciefied using the SPECIES1 flag, the second number is the atomic number of the atoms that have been specified using the SPECIES2 flag and so on.
+
+\plumedfile
+oap: METATENSOR ...
+      MODEL=soap_cv.pt
+      EXTENSIONS_DIRECTORY=extensions
+
+      SPECIES1=1-26
+      SPECIES2=27-62
+      SPECIES3=63-76
+      SPECIES_TO_TYPES=6,1,8
+...
+\endplumedfile
 
 \par Collective variables and metatensor models