Add tricontour

[mikeliux]

Description

Fixes #2715

This is a work in progress to implement tricontour.jl It is working already, with some improvements required, as described below.
Example:

using Random
Random.seed!(3)
# Adapted from existing tests
x = randn(Float32, (100,));
y = randn(Float32, (100,));
z = -sqrt.(x .^ 2 .+ y .^ 2) .+ 0.1 .* randn(Float32, (100,));
levels = 5;
fig3, ax3, trf = tricontourf(x, y, z, levels = levels);
scatter!(x, y, color = z, strokewidth = 1, strokecolor = :black)
Colorbar(fig3[1, 2], trf);
# Add contour lines (color not working properly yet)
trl = tricontour!(
    ax3, x, y, z,
    levels = levels,
    colormap=:reds,
    linewidth=3,  linestyle=:dashdot,
    );
Colorbar(fig3[1, 3], trl);
display(fig3)

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

Extended implementation Checklist

• Add tricontour methods, based on the existing tricontourf
• Update doctstring
• Update basic docstring for functions
• Plot 2D contour lines from unstructured data (at this point, colormap is not used
• Be able to pass desired plot attributes for lines!(), such as linewidth, linestyle,...
• Solve issue with colors, so plot can plot in colors
• Allow user to set a fixed color for all contour lines
• Allow user to define colorrange
• Fix issue with number of levels
• Add text labels
• Refactor common functions with contour.jl, contourf.jl and tricontourf.jl.