Improve adaptive stepsize for PANOC and ZeroFPR

[lostella]

PANOC and ZeroFPR require a step size gamma <= alpha/L for alpha < 1 and the smoothness constant L of the smooth term f. The backtracking procedure for gamma was not enforcing this: this fix updates the way the quadratic model for f is computed, and makes sure that alpha/gamma is used as diagonal Hessian in such quadratic model (and not 1/gamma).

In fact in some edge cases, the former behavior may have led the line search for tau to fail, later in the algorithms iteration. This fix makes sure that a more conservative gamma is selected in those cases, reducing the chance of line search failure.

[codecov]

Codecov Report

Merging #50 (5c794b5) into master (cf6f7e9) will decrease coverage by 0.39%.
The diff coverage is 100.00%.

❗ Current head 5c794b5 differs from pull request most recent head 6838308. Consider uploading reports for the commit 6838308 to get more accurate results

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   87.91%   87.51%   -0.40%     
==========================================
  Files          19       19              
  Lines         786      785       -1     
==========================================
- Hits          691      687       -4     
- Misses         95       98       +3     

Impacted Files	Coverage Δ
src/algorithms/forwardbackward.jl	95.52% <100.00%> (ø)
src/algorithms/panoc.jl	94.94% <100.00%> (ø)
src/algorithms/zerofpr.jl	94.04% <100.00%> (ø)
src/utilities/fbetools.jl	100.00% <100.00%> (ø)
src/algorithms/drls.jl	92.98% <0.00%> (-3.51%)	⬇️
src/utilities/iterationtools.jl	82.43% <0.00%> (-1.36%)	⬇️

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