Set a new default damp factor for exponential and gaussian damping
The default value of the damping factor was set without taking into account the propagation time. I have done some test on the spectrum of the benzoaldehyde molecule, to select the Limit of the damping factor at the end of the propagation. (see attached files). I choosed a value of 0.0001 for a good compromise between resolution and noise in the baseline.
- I have checked that my code follows the Octopus coding standards
-  I have added tests for all the new features added in this request.