Reorganize Reaction Ensemble

[jngrad]

Fixes #4155

Description of changes:

• move each class in the ReactionEnsemble into a separate source file
  • helps with separation of concerns and readability
  • Wang-Landau and its collective variables are in the same file due to coupling
• rename the namespace ReactionEnsemble to ReactionMethods
  • fixes the name collision with class ReactionEnsemble (used to be an issue in the class unit test)
  • chose ReactionMethods over the other candidates proposed in reaction_ensemble module contains several independent classes #3284 since the main class ReactionAlgorithm is currently still designed to use the SingleReaction class; the discussion in reaction_ensemble module contains several independent classes #3284 regarding class names remains open
• move the classes and unit tests to subfolder src/core/reaction_methods

[jonaslandsgesell]

Minor Comments

[jonaslandsgesell]

Looks good to me!