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/commands/test_lattice_region.cpp

@@ -20,7 +20,6 @@
 #include "lammps.h"
 #include "lattice.h"
 #include "region.h"
-#include "utils.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
@@ -631,7 +630,7 @@ int main(int argc, char **argv)
     MPI_Init(&argc, &argv);
     ::testing::InitGoogleMock(&argc, argv);
 
-    if (Info::get_mpi_vendor() == "Open MPI" && !LAMMPS_NS::Info::has_exceptions())
+    if (platform::mpi_vendor() == "Open MPI" && !LAMMPS_NS::Info::has_exceptions())
         std::cout << "Warning: using OpenMPI without exceptions. "
                      "Death tests will be skipped\n";