Major changes

1. In DensityMap.hpp, move the DensityMapType into an unnamed namespace, as it was not used anywhere except in this translation unit.
2. Remove the SE2 custom type and reduce code for the PointPair type in AlignRansac2d.hpp
3. Clean up the build system; now everything uses FetchContent instead of ExternalProject. All the configuration parameters for HBST are moved to the corresponding .cmake file. We modernized the eigen. cmake and removed CCACHE cause it was an overkill.