Reorganize & enhance linesearch algorithms

[ManifoldFR]

For more robustness we need a better linesearch procedure:

• [x] Refactor linesearch into own struct/algo that can be switched out
• [x] Implement: cubic interpolation (see minpack's linesearches) -- done in edaebfbb and 26699fb
• [x] Merge both Armijo backtracking (simple & interpolation) into single class
• [ ] Bracketing Wolfe search w/ support function derivative evaluation
• [ ] Linesearch diagnostic tools: plot interpolating polynomials in DEBUG mode

We can take inspiration from the implementations in Google Ceres: https://github.com/ceres-solver/ceres-solver/blob/master/internal/ceres/line_search.cc

[ManifoldFR]

Point 3 addressed by 57200519a46774decbf532727778bf5add873b01