GD with defaults and Armijo rule errors

[MargaretDuff]

Description

If you run GD without a step size method it defaults to the Armijo rule. If, after this, the Armijo rule throws this issue, it isn't very clear: https://github.com/TomographicImaging/CIL/blob/20979278acb858bba787b3d18c782eb520b43f75/Wrappers/Python/cil/optimisation/utilities/StepSizeMethods.py#L152-L156

The settings alpha and max_iterations correspond to internal Armijo rule settings, not global algorithm settings. To change it, the user needs to use a StepSizeMethod.

[MargaretDuff]

There are a few different options to fix this:

• Increase the default max_iterations in the Armijo rule
• Change the error message, to include instructions for defining the step size rule
• Change the default for GD to be 1/f.L or something similar?

[MargaretDuff]

For an example of this, see CIL Demos 01_optimisation_gd_fista.ipynb and change this step size to None, the default