Structure_Engine: Add MergeLoads method

#### Description:

Adding a method to be able to merge loads that can be merged.

Methods of merging:

- Any element load that has all of its properties except list of Objects equal, to merge into one load with all objects stored on it.
- Any element load that carries the same objects, and share all properties but the loadvector. Merge by making the final loadvector the sum of its merged parts.

Can be optionally used by Adapters when pulling loads from software that stores loads in a way that make them hard to overlap with the way BHoM operates.

@peterjamesnugent @JosefTaylor FYI