Print warning if incompatible alignment option chosen

[mvieth]

Users repeatedly report segmentation faults coming from Memory.h in the Eigen code (aligned_free/handmade_aligned_free). This is because PCL uses Eigen's handmade_aligned_malloc but user code does not use the corresponding handmade_aligned_free (or the other way around), due to chosen SSE/AVX flags and the resulting alignment requirements. PCL's CMake config handles this by automatically setting SSE/AVX options in the user project as they are used in PCL, but not everyone uses CMake. This commit adds a check within PCL's header files for this kind of incompatibility.

At first I considered solving theses incompatibilities automatically by setting EIGEN_MALLOC_ALREADY_ALIGNED and EIGEN_MAX_ALIGN_BYTES, but that would have been more complex and error-prone, so I decided only for checking and warning.

I would have preferred #warning which however is not available on MSVC. #pragma message is an option on MSVC, but this is too easily overlooked IMO. Users have the option to silence this by defining the PCL_SILENCE_MALLOC_WARNING macro (in case of false positive).

Most often, the culprit is the points member in pcl::PointCloud: Aligned memory is allocated within PCL and freed in user code (for example #6167 , #6164 , #6176 ). Maybe we should consider replacing Eigen's aligned_allocator in pcl::PointCloud with a custom allocator that does not decide the alignment based on SSE/AVX options.