Various Fixes

[varunagrawal]

1. Added a second test for DecisionTree::apply to show it doesn't enumerate all leaf options (so pruning actually works as expected).
2. Check for Apple Silicon before setting -march=native. If Apple system and architecture is arm64, then we don't set -march=native. Fixes Clang -march=native fails on Apple M1  #1057.
3. Remove spurious using statement so that the corresponding warning is resolved.

[varunagrawal]

Added a fix for 1057 because I got annoyed at that on the Macbook

[stefangachter]

Hi Varun, is this change not too strict? Actually, it limits the GTSAM_BUILD_WITH_MARCH_NATIVE flag to Apple architecture only. I was running into the issue under Ubuntu 18.04 (WSL2, Intel CPU). I expected to compile gtsam with -march=native when setting GTSAM_BUILD_WITH_MARCH_NATIVE=ON, but it did not add this compiler option. Thus, I got a segmentation fault when using GTSAM together in VIO package which was compiled with -march=native . To compile GTSAM properly, I set CMAKE_CXX_FLAGS="-march=native" and my issue was solved.

[varunagrawal]

The flag is by default off now but I see your point.

Can you please make a PR for the fix? I don't have cycles at the moment, but I'll gladly help land the fix.

[stefangachter]

I am not an expert here, but to my understanding, a general solution would be to add:

include(CheckCXXCompilerFlag)
CHECK_CXX_COMPILER_FLAG("-march=native" COMPILER_SUPPORTS_MARCH_NATIVE)
if(GTSAM_BUILD_WITH_MARCH_NATIVE AND COMPILER_SUPPORTS_MARCH_NATIVE)
    list_append_cache(GTSAM_COMPILE_OPTIONS_PUBLIC "-march=native")
endif()

And this should probably work with the most recent version of LLVM clang:
https://reviews.llvm.org/D119788
https://www.phoronix.com/news/LLVM-Clang-march-native-M1
Question to me is, what GTSAM should support.