initial commit for factor graph diff experiments with diff_factors

[nosmokingsurfer]

FGraphDiff module for experiments with differentiable factors

TODO list:

• abstract DiffFactor
• factor1Pose2_diff
• factor2Pose2_diff
• FGraphDiff
• FGraphDiffSolve
• derive and define diff-factor methods:
• evaluate_dr_dz()
• evaluate_dr_dx()
• evaluate_d2r_dx_dz()
• python bindings for FGraphDiff
• example project: mrob/src/FGraphDiff/examples/example_solver_2d.cpp - works with GN method
• example python project: /mrob/python_examples/FGraphDiff_2d.py- works with GN method
• resolve parent include headers ugly paths "#include "../FGraph/mrob/factor.hpp" - somehow using cmake probably?
• move whole gradient method from example into single funciton call
• update python bindings
• compose python training example

[nosmokingsurfer]

Hi, I will update file information comments, do clean up of EF methods adn etc and will remove redundant header includes where possible, but I have issues with headers in places where I do inheritance. In child class I need to include parent class header so the compiler can build object files for those classes. Linker works after all objects compiled - thus, compiler must know parent class declaration (method signature for example) to find them in corresponding parent object file.