Structure_Engine: MergePanels

#### Description:
Merge `Structure.Panel` (and/or any `IPanel`) which are coplanar, contiguous, and share a property into a single panel with the same property.

- [x] Write basic geometrical method which takes a list of `IPanels`, boolean unions the external edges and openings (separately), and outputs a list of (nominally fewer) `IPanels`(?) with no property assigned.
- [x] Write method which takes a list of panels, groups them by property, and calls the basic method on each group, assigning the property to the merged panels.
- [ ] implement [HasMergeablePropertiesWith](https://github.com/BHoM/BHoM_Engine/blob/b7b03a0785fc21a7dea21680925a4f94c760ef77/Physical_Engine/Query/HasMergeablePropertiesWith.cs) in order to compare properties
- [ ] Extend the method to optionally consider fragments and custom data as well as the surface property.