GeoStats.jl v0.18 includes better support for nested spatial correlation and geometric anisotropy.

Given spatial correlation functions, a nested correlation can be obtained via a linear model:

```
julia> γ₁ = GaussianVariogram(range=2.0, sill=3.0)
GaussianVariogram{Float64,Distances.Euclidean}
range: Float64 2.0
sill: Float64 3.0
nugget: Float64 0.0
distance: Distances.Euclidean
julia> γ₂ = ExponentialVariogram()
ExponentialVariogram{Float64,Distances.Euclidean}
range: Float64 1.0
sill: Float64 1.0
nugget: Float64 0.0
distance: Distances.Euclidean
julia> γ₁ + 2γ₂
NestedVariogram{2}
structures
└─GaussianVariogram(range=2.0, sill=3.0, nugget=0.0)
└─ExponentialVariogram(range=1.0, sill=1.0, nugget=0.0)
coefficients
└─1
└─2
```

In the multivariate case, the coefficients can be matrices, thus producing a linear model of coregionalization (LMC):

```
julia> [1.0 0.5; 0.5 2.0] * γ₁ + [3.0 0.1; 0.1 3.0] * γ₂
NestedVariogram{2}
structures
└─GaussianVariogram(range=2.0, sill=3.0, nugget=0.0)
└─ExponentialVariogram(range=1.0, sill=1.0, nugget=0.0)
coefficients
└─[1.0 0.5; 0.5 2.0]
└─[3.0 0.1; 0.1 3.0]
```

Geometric anisotropy can be produced with the `aniso2distance`

helper function, which now includes the rotation conventions of major commercial software (e.g. Datamine, Leapfrog):

```
help?> aniso2distance
search: aniso2distance
aniso2distance(semiaxes, angles; convention=:TaitBryanExtr)
A distance defined by an ellipsoid with given semiaxes and rotation angles.
• For 2D ellipsoids, there are two semiaxes and one rotation angle.
• For 3D ellipsoids, there are three semiaxes and three rotation angles.
Conventions
=============
Different rotation conventions can be passed via the convention option:
• :TaitBryanExtr => Extrinsic right-handed rotation by the ZXY axes
• :TaitBryanIntr => Intrinsic right-handed rotation by the ZXY axes
• :EulerExtr => Extrinsic right-handed rotation by the ZXZ axes
• :EulerIntr => Intrinsic right-handed rotation by the ZXZ axes
• :GSLIB => GSLIB software rotation convention
• :Leapfrog => LeapFrog software rotation convention
• :Datamine => Datamine software rotation convention (fixed to ZXZ axes)
Tait-Bryan and Euler conventions expect angles in radians. The other conventions expect them in degrees.
Default convention is :TaitBryanExtr.
Examples
==========
2D ellipsoid making 45ᵒ with the horizontal axis:
julia> aniso2distance([1.0,0.5], [π/2])
3D ellipsoid rotated by 45ᵒ in the xy plane:
julia> aniso2distance([1.0,0.5,0.5], [π/2,0.0,0.0])
```