Add more warnings and then eliminate their sources

[nlslatt]

Add this patch to magistrate and then clean up the sources of new warnings:

diff --git a/cmake/turn_on_warnings.cmake b/cmake/turn_on_warnings.cmake
index f492db2..5527255 100644
--- a/cmake/turn_on_warnings.cmake
+++ b/cmake/turn_on_warnings.cmake
@@ -14,6 +14,10 @@ endfunction()
 get_directory_property(hasParent PARENT_DIRECTORY)
 if(NOT hasParent)
   enable_cxx_compiler_flag_if_supported("-Wall")
+  enable_cxx_compiler_flag_if_supported("-Wextra")
+  enable_cxx_compiler_flag_if_supported("-Wshadow")
+  enable_cxx_compiler_flag_if_supported("-Wnon-virtual-dtor")
+  enable_cxx_compiler_flag_if_supported("-Wsuggest-override")
   enable_cxx_compiler_flag_if_supported("-pedantic")
   enable_cxx_compiler_flag_if_supported("-Wno-unknown-pragmas")
 endif()