Add support for distributional variables

[eliascarv]

Examples:

julia> gtb = georef((; a=[Normal(), Normal(1, 2), Normal(1, 3), Normal(2,3)]), CartesianGrid(2,2))
4×2 GeoTable over 2×2 CartesianGrid{2,Float64}
┌───────────────────────────────┬─────────────────────────────────────────┐
│               a               │                geometry                 │
│        Distributional         │               Quadrangle                │
│           [NoUnits]           │                                         │
├───────────────────────────────┼─────────────────────────────────────────┤
│ Normal{Float64}(μ=0.0, σ=1.0) │ Quadrangle((0.0, 0.0), ..., (0.0, 1.0)) │
│ Normal{Float64}(μ=1.0, σ=2.0) │ Quadrangle((1.0, 0.0), ..., (1.0, 1.0)) │
│ Normal{Float64}(μ=1.0, σ=3.0) │ Quadrangle((0.0, 1.0), ..., (0.0, 2.0)) │
│ Normal{Float64}(μ=2.0, σ=3.0) │ Quadrangle((1.0, 1.0), ..., (1.0, 2.0)) │
└───────────────────────────────┴─────────────────────────────────────────┘

julia> viewer(gtb)

julia> gtb = georef((; a=fill(Normal(0, 2), 4)), CartesianGrid(2,2))
4×2 GeoTable over 2×2 CartesianGrid{2,Float64}
┌───────────────────────────────┬─────────────────────────────────────────┐
│               a               │                geometry                 │
│        Distributional         │               Quadrangle                │
│           [NoUnits]           │                                         │
├───────────────────────────────┼─────────────────────────────────────────┤
│ Normal{Float64}(μ=0.0, σ=2.0) │ Quadrangle((0.0, 0.0), ..., (0.0, 1.0)) │
│ Normal{Float64}(μ=0.0, σ=2.0) │ Quadrangle((1.0, 0.0), ..., (1.0, 1.0)) │
│ Normal{Float64}(μ=0.0, σ=2.0) │ Quadrangle((0.0, 1.0), ..., (0.0, 2.0)) │
│ Normal{Float64}(μ=0.0, σ=2.0) │ Quadrangle((1.0, 1.0), ..., (1.0, 2.0)) │
└───────────────────────────────┴─────────────────────────────────────────┘

julia> viewer(gtb)

Plot 1:

Plot 2:

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (0c916e0) 0.49% compared to head (22aad54) 0.47%.

Files	Patch %	Lines
ext/viewer.jl	0.00%	12 Missing :warning:
ext/colors.jl	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #339 +/- ## ========================================= - Coverage 0.49% 0.47% -0.03% ========================================= Files 6 6 Lines 203 212 +9 ========================================= Hits 1 1 - Misses 202 211 +9 ```

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