Remove inherited self.blit attritube
Some necessary modifications to the Parameter and EELSCLEdge machinery to enable bidirectional communication between the widget and the value of the position Parameter when using enable_adjust_position

- Add twin_inverse attribute to Parameter
- EELSCLEdge.onset_energy becomes the main way to set the position of a CL edge. EELSCLEdge.energy_shift is now a twin of onset_energy and it might be removed in a future release.
- Enable bidirectional connection between the widget and the parameter value.
- Document twin_function and twin_inverse function in the User Guide