Pablitinho

Also working on VS 2017. But still missing include libs in cspare_extension (stuff) and missing #include in one header that I can not remember ...

In solver_cholmod is missing the include directories of cholmod and SuiteSparse_config

optimizable_graph.cpp was giving an error because missing: `#include `

It seems that it was not found by cmake. I added manually in VS.

I did it... But it is fine. Adding manually, it is working. 

FYI: This commit was possible to be compiled on VS2015 with cholmod activated. https://github.com/RainerKuemmerle/g2o/tree/20200410_git