Adding ability to simulate an empty domain (no panels)

[brookeslawski]

This PR adds the ability to simulate an empty domain (or atmospheric boundary layer simulation) in PVade, meaning that there are no panels or other structures in the domain. The purpose of this capability is primarily for benchmarking and testing of the fluid and thermal solutions in a simplified, canonical setting. A user may want to use this capability to characterize the flow and thermal fields (e.g. velocity, temperature, turbulence profiles) of the domain in a precursor simulation before adding in the panels and running again.

Feature adds:

To do:

• fix AttributeError: 'FSIDomain' object has no attribute 'structure' - it's looking for a structure mesh when it doesn't exist, only happens in 2d when structural_analysis=False -> need special treatment in MeshManager.py?
• capture error when there are too many cores for insufficient problem size (specifically when there is no structure) -> check and special treatment for calculating hmin_local
• add mesh refinement near the surface
• test in 3D