Unify PostScale methods for Muscles

[aymanhab]

Duplicated and identical PostScale methods for concrete Muscle subclasses are better moved to the base Muscle class.

[tkuchida]

IIRC we had this discussion and the rationale (whether still valid or not) was that all classes derived from Muscle may not want to update ofl and tsl in the same way. However, yes it seems reasonable to just have Muscle make this decision for any derived class. ⛄