Change the record structure of first order reaction and radioactive decay.

- rename records
- change input files using reactions and decays
Improve comments, rename members, improve readability.
Introduce LinearODESolver in the reaction term.

- add interface for solving a system of linear first order ODEs
- hide PadeApproximant and analytic solution under that interface
- add record ode_solver in reaction term
Rename reaction classes FirstOrderReaction, RadioactiveDecay.
Correction in analytic solution in LinearODESolverAnalytic.
Cleanup in reactions, edit comments.