Refactor reaction method constraints

[jngrad]

Description of changes:

• unit test reaction constraints
• make reaction constraint fields private
• make invalid constraint parameters raise a ValueError
• make it safe to change a reaction constraint from cylindrical to slab
• make it possible to remove a reaction constraint

[jngrad]

@reinaual This isn't a perfect solution, but there is no need to spend more time on this part of the code for now. The long-term plan is to extract the reaction constraint logic from the ReactionAlgorithm class to improve extensibility and separation of concerns. Shouldn't be too hard now that the logic is fully private. An enum was the quickest way to make the code safely unit testable.