Fix and polish Nesterov acceleration tooling and fast proximal gradient implementation

[lostella]

This PR:

• Polishes, expands, documents the tooling around Nesterov acceleration
• Fixes an off-by-one mistake in the sequences definitions (which would be fine if the first element in the sequence were "burned" because x_{-1} == x{0}, but the implemented algorithms always do the first extrapolation after the first step is taken)
• Allows for custom extrapolation sequences to be used in FastForwardBackwardIteration

Also:

• De-randomize two tests which are otherwise giving issues across different Julia versions

[codecov[bot]]

Codecov Report

Merging #58 (17cb237) into master (238dfd2) will decrease coverage by 0.33%. The diff coverage is 84.84%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   89.73%   89.39%   -0.34%     
==========================================
  Files          22       22              
  Lines         945      962      +17     
==========================================
+ Hits          848      860      +12     
- Misses         97      102       +5     

Impacted Files	Coverage Δ
src/accel/nesterov.jl	81.81% <81.48%> (-10.49%)	:arrow_down:
src/algorithms/fast_forward_backward.jl	97.50% <100.00%> (-0.18%)	:arrow_down:

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