Allow for specifying the lower bound of the Fourier transform. Also simplifies a bit the code. This is controlled by the variable PropagationSpectrumMinEnergy.
Closes #146 (closed)
Adding a variable PropagationSpectrumMinEnergy to control the minimum energy in Fourier transforms.
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.