graphslam-engine: Cleanup and Fix code bugs

[bergercookie]

@jlblancoc, @EduFdez

Final product should address the following:

• [x] Remove hwdrivers lib/app dependency
• [x] Cut down on the size of graphslam-engine-demos directory
• [x] Reduce size of application-page, add details about library implementation, application usage in a seperate external guide
• [ ] Investigate trajectory zig-zags
• [ ] Fix RGB-D dataset handling
• [ ] Deal with synchronization issue when optimize_on_second_thread flag is True
• [x] Modify the structure of library and application files. Have all the methods defined in the library files and just make the necessary calls from the application - Split the execGraphSlamEngine method