Add the possibility to set a different frequency for each sensor

[GiulioRomualdi]

Thanks to this PR it will be possible to set different frequencies for each sensor exposed in the device. To do that I:

• removed the polling thread and used the callback provided by xsens to get the data
• Add optional parameters to specifically set the period of each sensor exposed by the IMU. If the parameter is not passed the xsensmt_period is used. This means that this implementation is retro compatible with the already existing configuration files
• I updated the readme

cc @traversaro @HosameldinMohamed

[GiulioRomualdi]

Minor comment. Can you add a line in the CHANGELOG?

Done

[HosameldinMohamed]

• removed the polling thread and used the callback provided by xsens to get the data

Thanks, @GiulioRomualdi for this! I will later add the position support to the MAS interface according to #39