OpticalEncodersDrift: Added features regarding folder architecture and date/time for saved files

[gsisinna]

This PR includes improvements and additions that make saving files more organized and intuitive. With the following code, a nested tree structure is created, which is useful for automating the optical encoders drift test without overwriting files (as was the case before).

Example:

results
├── iCubErzelli02
│   └── encoders-icub_30052022
│       └── encDrift
│           ├── encDrift_plot_left_arm_30052022_1612.txt
│           ├── encDrift_plot_right_arm_30052022_1600.txt
│           └── encDrift_plot_right_arm_30052022_1614.txt
└── RobotName
    └── encoders-icub_30052022
        └── encDrift
            ├── encDrift_plot_left_arm_30052022_1617.txt
            └── encDrift_plot_right_arm_30052022_1619.txt

For greater accuracy related to reporting, date and time of test execution and file generation have been included, and for folder names the value is read directly from the environment variables ($YARP_ROBOT_NAME).

[Nicogene]

Merged, thanks!