integrate platform sub-namespace into source code and documentation

this updates function calls to functions that have been moved from
the utils namepsace or the Info class to platform::

unittest/cplusplus/test_lammps_class.cpp

@@ -325,7 +325,7 @@ TEST_F(LAMMPS_kokkos, InitMembers)
 TEST(LAMMPS_init, OpenMP)
 {
     if (!LAMMPS::is_installed_pkg("OPENMP")) GTEST_SKIP();
-    if (Info::get_openmp_info() == "OpenMP not enabled") GTEST_SKIP();
+    if (platform::openmp_standard() == "OpenMP not enabled") GTEST_SKIP();
 
     FILE *fp = fopen("in.lammps_empty", "w");
     fputs("\n", fp);