Result_Engine: Generalise Envelop methods

#### Description:

Current envelop methods are specifically targeting `BarForces`. This is something we would want to change to target any applicable `IResult`.

One solution to this would be for all IResults that want this functionality to have a method called "EnvelopParameters" or similar, a method that returns the properties of to be evaluated, for example FX, FY, FZ, MX, MY and MZ for Barforces.

This can then be extracted and stored by the Result_Engine and used to be able to envelop any result type.