While it is the case for the existing official Skills, Skill implementations shouldn't be required to conform to a strain based approach.
There are other valid approaches to calculating skill difficulty that can be supported by abstracting the strain logic into its own StrainSkill class.
Although this isn't necessary for existing official rulesets and calculators, custom calculators can have use cases for accessing mods in difficulty calculation.
For example, accounting for the effects of visual mods.
