IfcSchema declaration_by_name to be safe

Couldn't find if we actually use it anywhere in the code but better to keep it safe, especially because it's exposed to Python and by accident user might provide some too bit integer instead of a string.

src/ifcparse/IfcSchema.h

@@ -496,7 +496,7 @@ class IFC_PARSE_API schema_definition {
     }
 
     const declaration* declaration_by_name(size_t name) const {
-        return declarations_[name];
+        return declarations_.at(name);
     }
 
     const std::vector<const declaration*>& declarations() const { return declarations_; }