Add linear optimization to Optimize Recoils or Fluence

[tpitkanen]

Issue: #193

TODO:

• [ ] _generate_mev_to_nm_function(): handle the case where there are really short peaks at surface or between valid peaks (ignore missing peaks or use the longest streak of non-zeroish peaks)
• [ ] unit tests
• [ ] add an option to get the starting solution from user

Future TODO:

• [ ] remember which combination of NSGA-II/Linear and recoil/fluence is selected (not as simple as saving the other settings because the settings are saved using PropertySavingWidget, which doesn't work well on individual variables). As a quick fix, linear optimization could be the new default selection.
• [ ] add support for fluence optimization (currently only recoil optimization is supported)
• [ ] multi-thread initial espe runs (ElementSimulation.calculate_espe() uses multiple recoils for some reason, including self.optimization_recoils[0]. Check if a single recoil can be safely used instead.)
• [ ] customizable shape (number of peaks, rectangular or not, peak at surface or deeper)
• [ ] selectable number of iterations

[tpitkanen]

This isn't entirely polished yet (missing some edge cases, a bit clunky settings, no tests), but it's usable.