CMake Error at CMakeLists.txt:7 (find_package):
By not providing "FindEigen3.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "Eigen3", but
CMake did not find one
Could not find a package configuration file provided by "Eigen3" with any
of the following names:
Eigen3Config.cmake
eigen3-config.cmake
Add the installation prefix of "Eigen3" to CMAKE_PREFIX_PATH or set
"Eigen3_DIR" to a directory containing one of the above files. If "Eigen3"
provides a separate development package or SDK, be sure it has been
installed.对于此类问题，可能是几种情况。
一、没有安装相应的依赖包，这些包大多数是独立于ROS的。
解决方法：从github上下载对应的包，安装在catkin_make下，可以自己新建一个文件夹来装这些包，注意，不能放在src下。
之后按照github上的readme或者install进行安装。
对于一般的包，安装的方法是,首先cd进入下载的包,然后
mkdir build && cd build
cmake ..
make
sudo make install
二、对于如下面写的曾经装载了Eigen的情况。在这种条件下，先进入你的工作空间
cd ~/catkin_make
然后locate FindEigen3.cmake
找到了位置后，cp 刚刚找到的位置 src/你报错的package/cmake
这样就可以解决了。