[migrate] rename method to grading_type; add default for curved
[sample] change method to grading_type
[spec/lib/model] rename method to grading type
[view] rename method to grading type
[controller] change strong params to grading_type
[locale] add grading type
[helper] refactor helper
[spec/feature] rename method to grading type
[model] refactor grading method
[factory] add grading type to grading method factory
[lib/grading] change lookup from method to grading type