Change Base unit from k$\lambda$ to $\lambda$

[iancze]

Feature branch to convert base unit from k $\lambda$ to $\lambda$.

When merged into main,

• closes Base unit for spatial frequency #223
• closes mpol.utils.convert_baselines and mpol.utils.broadcast_and_convert_baselines duplicate visread functionality #227
• restructures the mock dataset to be generated on the fly
• closes GridCoords constructor with @properties and @cached_properties #187 by updated Updated #187: GridCoords constructor with @properties and @cached_properties #250

• Standardized nomenclature of {class}mpol.coordinates.GridCoords and {class}mpol.fourier.FourierCube to use sky_cube for a normal image and ground_cube for a normal visibility cube (rather than sky_ for visibility quantities). Routines use packed_cube instead of cube internally to be clear when packed format is preferred.
• Modified {class}mpol.coordinates.GridCoords object to use cached properties #187.
• Changed the base spatial frequency unit from k$\lambda$ to $\lambda$, addressing #223. This will affect most users data-reading routines!