Add a function in Database abstract class to get the number of data modalities in the database

include/aidge/data/Database.hpp

@@ -33,6 +33,13 @@ public:
      */
     virtual std::size_t getLen() = 0;
 
+    /**
+     * @brief Get the number of modalities in one database item
+     * 
+     * @return std::size_t 
+     */
+    virtual std::size_t getNbModalities() = 0;
+
 };
 } // namespace Aidge