GetModelState.srv with a Header (Timestamp)

Hi

I anhanced the GetModelState service with header information because it is important for the client to have a timestamp with the model.

Greetings
Markus
stick to the same `if` `else` style as in the rest of the file.

    if (condition) // With the space after the if
    {
      ...
    }
    else
    {
      ...
    }