Signal provider root name is hard coded

As per title, the signal provider root name is hard coded (line 66)

https://github.com/ami-iit/robot-log-visualizer/blob/1a61360411a0100529c8011c8d9fb6273f62bbe4/robot_log_visualizer/file_reader/signal_provider.py#L32-L70

For our use cases it be useful to use the filename of the loaded `.mat` as the `root_name`, but have no idea of other applications (and neither how much is difficult to implement it)

Start in robot_log_visualizer/file_reader/signal_provider.py around the linked lines and inspect how the signal provider root name is assigned when a .mat file is loaded. Confirm how the filename should become root_name without breaking other uses, then verify the visualizer still loads the file correctly.

Written by the indexing model from the issue text.