Change behavior when searching for eigen3
Change behavior when searching for eigen3

By default, try to use CMake FIND_PACKAGE() instead of pkg-config to find
eigen3. If not found, by default, shown an error msg, and allow the user
to enable downloading it. This changes the previous behavior where the
lib was automatically downloaded without asking the user.

cc: #591

MRPT/mrpt,MRPT/mrpt,MRPT/mrpt,MRPT/mrpt,MRPT/mrpt