Abstraction layer around vtk export

[KnutAM]

closes #278 (replaces #691, xref #686)

Following the discussion in #691, this PR introduces an abstraction layer for exporting from Ferrite to VTK. With this PR, no functions from WriteVTK.jl should be called directly by the user. (If still needed for special/custom reasons, it is possible by accessing the internal methods and or the vtk field in VTKFile)

Using vtk_grid with an AbstractGrid or AbstractDofHandler will error telling how to upgrade

Main motivations

1. Avoid re-exporting and extending methods from WriteVTK. Makes dispatch and function owner ship more clear and flexible
2. Provides infrastructure for backend-agnostic export by allowing users to define their own backends (see e.g. Abstraction layer around vtk export #692 (comment) below)

The new workflow is

VTKFile(filename, grid_or_dh; kwargs...) do vtk
    write_solution(vtk, dh::DofHandler, u)
    write_projection(vtk, proj::L2Projector, vals)
    write_cell_data(vtk, celldata)
    write_node_data(vtk, nodedata)
    Ferrite.write_cellset(vtk, grid, [setnames])
    Ferrite.write_nodeset(vtk, grid, setname)
    Ferrite.write_constraints(vtk, ch::ConstraintHandler)
    Ferrite.write_cell_colors(vtk, grid, cell_colors)
end

or alternatively

vtk = VTKFile(filename, grid_or_dh; kwargs...)
# write ....
close(vtk) # Overload of Base.close

For writing paraview collections, the workflow is

pvd = VTKFileCollection(filename, grid_or_dh; kwargs...)
for t in range(0, end_time, num_steps)
    # calculate e.g. the dof-vector u and cell data s
    addstep!(pvd, t, [grid_or_dh]) do vtk
        write_solution(vtk, dh, u)
        write_cell_data(vtk, s, "stress")
    end
end
close(pvd)

Alternatively,

pvd = VTKFileCollection(filename, grid_or_dh; kwargs...)
for (n, t) in pairs(range(0, end_time, num_steps))
    # calculate e.g. the dof-vector u and cell data s
    vtk = VTKFile(string(filename, "_", n), grid_or_dh)
    write_solution(vtk, dh, u)
    write_cell_data(vtk, s, "stress")
    addstep!(pvd, vtk, t)
end
close(pvd)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.86%. Comparing base (2ca4f94) to head (d6a199e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #692      +/-   ##
==========================================
+ Coverage   93.56%   93.86%   +0.29%     
==========================================
  Files          36       36              
  Lines        5272     5313      +41     
==========================================
+ Hits         4933     4987      +54     
+ Misses        339      326      -13     

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[koehlerson]

having something like write_projected sounds very good to me at least from a new user perspective it seems quite convenient to have this

[KnutAM]

Not sure why the mac CI fails, seems unrelated to changes but I don't have a mac so cannot test locally.
Edit: Is unrelated, happens in other PRs too (xref #914 / bbc202a)