fix ParameterAlreadyDeclaredException in _on_heartbeat_timeout

[y-bachmann]

When a bond times out and after that a new bond is created which also times out, the following exception occurs:

Exception in thread Thread-121:
Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.10/threading.py", line 1378, in run
    self.function(*self.args, **self.kwargs)
  File "/opt/ros/humble/lib/python3.10/site-packages/bondpy/bondpy.py", line 85, in _on_timer
    self.on_timeout()
  File "/opt/ros/humble/lib/python3.10/site-packages/bondpy/bondpy.py", line 187, in _on_heartbeat_timeout
    self.node.declare_parameter(Constants.DISABLE_HEARTBEAT_TIMEOUT_PARAM, False)
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/node.py", line 367, in declare_parameter
    return self.declare_parameters('', [args], ignore_override)[0]
  File "/opt/ros/humble/local/lib/python3.10/dist-packages/rclpy/node.py", line 490, in declare_parameters
    raise ParameterAlreadyDeclaredException(parameters_already_declared)
rclpy.exceptions.ParameterAlreadyDeclaredException: ('Parameter(s) already declared', ['/bond_disable_heartbeat_timeout'])

By guarding against ParameterAlreadyDeclaredException this can be easily handled.