use of non-threadsafe std::cout in examples, such as fig_3_03

[jnorwood]

The examples often use std::cout inside tasks, and the output can get pretty jumbled.
For example, fig_3_03 output of

std::cout << "first node received: " << in << std::endl;

I've been changing these to, for example
`
std::stringstream ss;

  ss << "first node received: " << in << std::endl; 

  std::cout << ss.str();

`
This seems to work, although it isn't clear to me that even it is guaranteed.
So, I'm adding a cout_locked function that should be guaranteed in my examples

`
tbb::spin_mutex mylock;

void cout_locked(const std::string &ss)

{

tbb::spin_mutex::scoped_lock smut(mylock);

std::cout << ss;

}
`
and so the code above becomes

`
std::stringstream ss;

  ss << "first node received: " << in << std::endl;

  //std::cout << ss.str();

  cout_locked(ss.str());

`

The problem with the current output is easy to see if the fig_3_03 example is modified to send more messages in the test

`
// step 4: send messages

for (int i=0;i<100;i++)

my_first_node.try_put(i);

`