Add high level methods for sliceplot and lineplot

[pjaap]

This is based on WIAS-PDELib/GridVisualizeTools.jl#20

Implementation of high level methods for line and slice plotting.
The user only has to provide a generic line or plane definition to obtain a plot.

The new coordinate systems are created by isometric rotation and therefore the angles are invariant.

Examples:

grid = simplexgrid(-1.0:0.01:1.0, -1.0:0.01:1.0)
f = map(x -> x[2]*x[1], grid)
line = [1,1,0] # plot along x + y = 0
lineplot(grid, f, line, Plotter=GLMakie)

grid = simplexgrid(-1.0:0.1:1.0, -1.0:0.1:1.0, -1.0:0.1:1.0)
f = map(x -> x[2]*x[1], grid)
plane = [1,1,0,-1.51] # plot along x + y + 0z = 1.51
sliceplot(grid, f, plane, Plotter=GLMakie) 

Note that the intersection is about of size 2 x 0.75 in the new transformed coordinates.

There is a bug in marching_tetrahedra, creating some holes in the plot if the tetrahedra are sliced along the facets:

plane = [1,1,0,-1.5] # plot along x + y + 0z = 1.5

[j-fu]

Ah cool - this seems to work without need to interfere with the plotting backend code.

As for the naming: in fact, these are still scalarplots after all.

An alternative would be to just use the scalarplot method and to add a hyperplane
kwarg. So we could have

scalarplot(grid, f; hyperplane=[1,1,0],Plotter=GLMakie)

[pjaap]

Sorry for the late answer. I somehow missed it.

Thanks for the positive feedback. I like your naming suggestion to keep it as scalarplot.
However, a hyperplane is usually used for more than three dimensions, right?
We already have the planes kwarg, which is too similar to plane.

I think the cleanest solution is slice = Plane(a,b,c,d) for an explicit dummy class Plane. And Line(a,b,c) for the line plot.
Then, we can elegantly document the plane equation coefficients.

[j-fu]

IMHO "hyperplane" is the dimension independent name for an n-1 dimensional affine subspace.
So a line is a hyperplane in 2D, and a plane is a hyperplane in 3D. I am really a fan of dimension independent naming.