Add coaxial junction to join two coaxial pipes together

[mattfalcone1997]

This junction allows the coaxial tubes of different geometries to be joined. In this implementation, both the solid region and fluid regions are connected, with the flow channels are optionally joined. This allows components such as pumps may be added. Currently, the flow junctions are assumed to have no momentum loss, which can be improved in the future if necessary.

Considerations

• Coupling solid regions with different geometries is not straightforward here, as existing coupling components require a one-to-one element mapping. This significantly restricts geometry variation.
  • This can be remedied by implementing a slightly different variation of the 2D coupling component.
  • This can be added if it is found to be necessary.

Testing

• Tests verifying solid coupling
• Tests ensuring continuity of mass flow rate
• Tests to show that error are raised correctly
• Await merge of coaxial pipe