Minor Compilation Issues When Using as Standalone Library

[nocholasrift]

Hi there! I was compiling some code in C++17 that uses the GCOPTER headers for a current research project, and noticed 2 compilation errors that I figured I'd bring up:

1. gcopter.hpp uses methods in the geo_utils namespace, but doesn't directly or indirectly #include "geo_utils.hpp". Adding the include seemed to fix the problem.
2. The filterLess struct operator() function in geo_utils.hpp isn't const. Changing the function header to the following allowed me to compile successfully:
   inline bool operator()(const Eigen::Vector3d &l, const Eigen::Vector3d &r) const;

Thanks for sharing this framework, it's very nicely done!