optimize module, string and system update

• Currently the modules, strings and system don't know when a constituent has changed.
• using __setattr__ like pvcells would trigger too many recalculations.
• need update methods, similar to setSuns()
• currently update is manual using calcMod(), calcString() and calcSystem() but output has to be assigned to attributes, the models don't update themselves.
• these is error prone and leads to confusion, if the user thinks they've updated but haven't actually applied the changes.
• needs to be flexible, in case the user decides to update many fields, then don't need to recalculate after each one.