Disable saveToFile if `auto_save` or save_periodically is enabled

[traversaro]

If a user requested the save to performed by the destructor (with auto_save) and/or periodically (with save_periodically), then it would be great if the saveToFile method (called by the user) would return an error, otherwise it may happen that there are race condition between the user code and the other threads. We had this problem with @Gianlucamilani.

[Nicogene]

The only problem is that saveToFile is the same function invoked buy the save thread and by the destructor when save_periodically and auto_save are respectively enabled, I don't know how to intercept that this is a call from the user code and not a call from BufferManager

[traversaro]

I would rename the function used by the saveThread and the destructor to saveToFileInternal (or some smarter name) and keep the saveToFile public function for being called by external users, that will just call saveToFileInternal after any check we like (such as the one described in the issue).

[S-Dafarra]

I think that with #199 now the user can know when the buffer is about to be saved. By the way, from the robometry side it should "safe" calling saveData while a periodic save is called (I guess). I believe there are cases in which it is useful to have all three saving methods available anyhow.