Introduce OctaminimalGaussianArray, smaller, faster but less accurate

[milankl]

Motivated by #594 this reduces the number of grid points on the OctahedralGaussianGrid by starting at 4 points around the pole instead of 20. At nlat_half = 24, the default resolution, this yields 2400 grid points instead of 3168, i.e. -25%, hopefully making the parameterizations 25% faster, and the whole model by 12.5%-ish as parameterizations are currently more than 50% of total runtime.

Number of grid points per ring are (old, new)

julia> [(16+4i, 4i) for i in 1:24]
24-element Vector{Tuple{Int64, Int64}}:
 (20, 4)
 (24, 8)
 (28, 12)
 (32, 16)
 (36, 20)
 (40, 24)
 (44, 28)
 (48, 32)
 (52, 36)
 (56, 40)
 (60, 44)
 (64, 48)
 (68, 52)
 (72, 56)
 (76, 60)
 (80, 64)
 (84, 68)
 (88, 72)
 (92, 76)
 (96, 80)
 (100, 84)
 (104, 88)
 (108, 92)
 (112, 96)