`using namespace std` in headers

[mrj10]

Ramulator's header files have lots of using namespace std; in them.

This poisons the global namespace of any .cpp file which #includes Ramulator headers with tons of symbols from the standard library.

Best practice is to avoid using namespace in header files, use it only in .cpp files as appropriate, and be explicit with std:: in header files.

See e.g. here

Can we remove using directives from the headers?