Check identical timestamps before using IMU data

Solved in this MR !468 (merged)

If timestamp precision is not sufficient, two identical times could be found in the external sensor data files. It leads to following crash from gtsam:

terminate called after throwing an instance of 'std::runtime_error'
  what():  PreintegratedImuMeasurements::integrateMeasurement: dt <=0
[1]    1376071 IOT instruction (core dumped)  ./LidarView

A time check should be added before using IMU data.

A previous similar error

FYI @gatien.ferret @timothee.couble