Simplify default vertical coordinate spacing to equi-distant with sigma

[milankl]

julia> spectral_grid = SpectralGrid(trunc=21,Grid=FullGaussianGrid,nlev=8)
SpectralGrid:
├ Spectral:   T21 LowerTriangularMatrix{Complex{Float32}}, radius = 6.371e6 m
├ Grid:       32-ring FullGaussianGrid{Float32}, 2048 grid points
├ Resolution: 499km (average), 625km × 616km (Equator)
└ Vertical:   8-level SigmaCoordinates

julia> spectral_grid.vertical_coordinates
8-level SigmaCoordinates
├─ 0.0000  k = 0.5
│× 0.0625  k = 1
├─ 0.1250  k = 1.5
│× 0.1875  k = 2
├─ 0.2500  k = 2.5
│× 0.3125  k = 3
├─ 0.3750  k = 3.5
│× 0.4375  k = 4
├─ 0.5000  k = 4.5
│× 0.5625  k = 5
├─ 0.6250  k = 5.5
│× 0.6875  k = 6
├─ 0.7500  k = 6.5
│× 0.8125  k = 7
├─ 0.8750  k = 7.5
│× 0.9375  k = 8
└─ 1.0000  k = 8.5