Maths in FISTA and ISTA documentation is incorrect/inconsistent

[lauramurgatroyd]

Description

Current status of ISTA docs:

After discussion with @MargaretDuff we agreed

• superscripts should be replaced with subscripts to be consistent with FISTA docs
• we should remove references to alpha_k as it is overcomplicated, not referenced in FISTA docs, and varying step sizes are mentioned instead under the step_size parameter

Current status of FISTA docs:

• Needs improving to be in line with FISTA paper
• t_k+1 is calculated but not used - the t__{k-1} in the last equation should be t_{k+1}
• y_k definition is incorrect and should be removed
• definition of x_k+1 in update() should instead be x_k and that defn should be used in class docs
• Add definition of x_0 and y_0 and t_1

Environment

import cil, sys
print(cil.version.version, cil.version.commit_hash, sys.version, sys.platform)