New save_maker_chain function

[aelligp]

I added a new function to save a line of points to a vtk file to track e.g. the evolution of the surface over time. Our use case is to track the marker chain.

save_marker_chain(fname::String, cell_vertices::LinRange{Float64}, h_vertices::Vector{Float64})

Save a vector of points as a line in a VTK file. The X and Y coordinates of the points are given by `cell_vertices` and `h_vertices`, respectively.
The Z coordinate is set to zero as we are working in 2D.

## Arguments
- `fname::String`: The name of the VTK file to save. The extension `.vtk` will be appended to the name.
- `cell_vertices::LinRange{Float64}`: A range of X coordinates for the points.
- `h_vertices::Vector{Float64}`: A vector of Y coordinates for the points.

## Example
cell_vertices = LinRange(0.0, 1.0, 10)
h_vertices = [0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9]
save_marker_chain("Example", cell_vertices, h_vertices)

Also features some errors fixes and minor changes to the testing framework

[albert-de-montserrat]

Great thanks!