support arbitrary search method in fast_gicp

[themightyoarfish]

There is nothing in the code that assumes more than the pcl::search::Search interface of the search objects. So one might use others such as OrganizedNeighbor which is much faster for organized clouds.

The code distinguishes between source and target methods, but since swap() is called on their pointers, I think in practice only PointSource == PointTarget is supported.

[koide3]

Thanks for your PR! I think this functionality is reasonable and nice. I'll take a quick look at the updates.

[koide3]

Because PCL installed via apt-get causes segfaults with march=native (Possibly, the CI failure is caused by this flag), I would like to make this optional like:
https://github.com/koide3/ndt_omp/blob/0852c95360d1b0d29745e7eae7a57c0950de695b/CMakeLists.txt#L7

[themightyoarfish]

Actually i think we should use PCL_DEFINITIONS here instead of manually specifying any flags, as any PCL client project has to be compiled with the same flags to prevent such crashes.

[themightyoarfish]

Added that in 672e71f

[themightyoarfish]

I've restored align.cpp and added the SMID flags based on PCL, which I believe is necessary as you don't know how the user installed PCL.

I also realized that OrganizedNeighbour does not in fact work for all organized point clouds, but just for image-like ones such as from depth cameras. Still useful to have this templated though I think, hopefully some day PCL will add an organized search method for rotating LIDARs.

[koide3]

Thanks a lot for your contribution. All changes look good now. I'll merge this PR once the small conflict caused by #134 is resolved.

[themightyoarfish]

Fixed.

[koide3]

Thank you a lot!