This commit adds a simple version of numpy_eigen, copied from
gtborg/numpy_eigen commit 255c09efb82496, and with a fix released in the commit
9a75383733b3dc4bc2bb0649053949ad2bec9326 of Scheizer-Messer/numpy_eigen
(https://github.com/ethz-asl/Schweizer-Messer/tree/master/numpy_eigen)
Conflicts:
CMakeLists.txt
gtsam/CMakeLists.txt