Implement line search for shell minimization

It may help to speed up convergence. Or indeed to compute the energy at a full step, half a step and the starting position, then approximate by a parabola.

No file, test, or entry point is named. Start by locating the shell-minimization implementation and tracing how energy and convergence are currently evaluated; clarify the intended line-search behavior and success criteria before choosing among the suggested step evaluations. Done should include a defined, tested line-search approach that improves or preserves minimization convergence.

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