IMUPreintegrator::update() function A matrix initializated with indentity() maybe is not correct.

[jdcllh]

hello,
thanks your great implementation!
for A matrix initialization in IMUPreintegrator.cpp, line 91, if you initialized the A matrix with indentity, then i think, you should also let A.block<3,3>(6,3), A.block<3,3>(6,0) and A.block<3,3>(3,0) to zero, as show below Forster supplementary paper: