Structure_Engine: Add method to add suffix to ResultCase

#### Description:

Found it useful when multiple side study models have been run for a case with the same name to add a suffix to the result case to be able to differentiate between the models.

A method taking for example a BarForce and a string, that returns the same barforce, but with the ResultCase modified to include the additional string on the end of the result case.

Alternative is to add model name/information to the result objects themself, but a bit caution of the size of the obejcts, as that would weigh them down a bit.

@peterjamesnugent @JosefTaylor thoughts?