Revert back to std::cout from printf

[elindgren]

@brucefan1983 Would it be possible to revert back to using std::cout for logging/prints instead of printf? Capturing the output from nep.cpp becomes difficult if it is written with printf, as it does not write to the same output stream as std::cout.

Here is an example of the difference between the two:

Old:

std::cout << "Use the NEP2 potential with " << paramb.num_types << " atom type(s).\n";

New:

printf("Use the NEP2 potential with %d atom type.\n", paramb.num_types);

[brucefan1983]

sure, you do this or I do it?

[brucefan1983]

I have been mixing iostream and stdio a lot, which is bad. It is better to unify to use the C++ way.

[elindgren]

I can do it :)

[brucefan1983]

ok, you can make a PR. I will add you as developer for this repo.