using namespace in header files

[PovelikinRostislav]

It's a bad practice to inject using namespace into header files like in sycl_base.hpp:

using namespace std;
using namespace cl::sycl;
using namespace cl::sycl::access;

I'll submit a PR fixing this issue and other polishing stuff (absence of used headers in a list of include files in some examples) as soon as I get some free time.