Refresh Token of an existing application
refresh_token_app(client_id, client_secret, refresh_token)a character string specifying the 'client_id' of the registered (existing) Fitbit application
a character string specifying the 'client_secret' of the registered (existing) Fitbit application
a character string specifying the 'refresh_token' of the registered (existing) Fitbit application
a named list that includes access_token, expires_in, refresh_token, scope, token_type, user_id
A registered Fitbit application has a time limit of 8 hours. Therefore, the user has to refresh the token after the expiration using the 'client_id', 'client_secret' and 'refresh_token' that it's available for the registered application. Based on the Fitbit API Documentation "After the Access Token expiration time has passed your requests will receive a 401 HTTP error. When this happens, your app should use the Refresh Token to get a new pair of tokens"