An incorrect variable name caused building failure.

[AphroraC]

Hello!
Thank you for making this remarkable poject FOSS!

There is an wrong var causing build failure:

in src/gtsam_points/optimizers/levenberg_marquardt_ext.cpp 72:12

return gfg.error(x);

I changed it into "return graph.error(x)" and passed building.
Maybe you could check it out. :)

[koide3]

Thanks for reporting the bug. I just fixed it!