Add support for setting sensor parameters externally

[alexmora-sr]

First of all, congratulations on your paper publication! Targetless calibration for LiDAR-IMU systems is very interesting for a number of applications. I have been working on a LiDAR-IMU calibration pipeline myself, and as you mention in your paper, there is a lack of open-sourced packages for this problem. I would personally like to thank you for your contribution to the community.
I have been using your tool to get a reliable calibration, and plan to use it to benchmark my own algorithm.

I would like to contribute to your project by allowing the user to externally set the gyroscope/accelerometer noise densities, LiDAR noise and IMU rate, so that the optimization weights may be computed according to the user's specific sensors.

The proposed changes in this pull request include:

• Allow providing said parameters via the main calibration script "calib.sh" and the launch file "launch/licalib_gui.launch".
• Implement default values in "calibration.hpp", and a way to check and update the optimization weights in "src/ui/calib_helper.cpp".

Tested in Ubuntu 18.04