renaming main logic files. Added warning message of the removal and include passthrough. Renamed includes in the source files. Ran Clang-Format (#946)

Header rename

tests/assertion_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <gtest/gtest.h>
-#include <gsl/gsl_assert> // for fail_fast (ptr only), Ensures, Expects
+#include <gsl/assert> // for fail_fast (ptr only), Ensures, Expects
 
 using namespace gsl;