Paur task1

[amartinhuertas]

To describe ... @ericneiva @fverdugo @paurierap involved

Related to #16

[codecov-commenter]

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@@                Coverage Diff                 @@
##             gsoc2021-master      #18   +/-   ##
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[fverdugo]

@paurierap I have refactored the conversions significanly. The goal of this refactoring is:

• Avoid type-piracy. By defining GeometryBasiscs.Mesh(::Grid) we are doing so-called type piracy since neither Mesh nor Grid are defined in this package. I have used another function name to fix this: to_plot_mesh so that grid |> to_plot_mesh returns the mesh ready to be visualized.
• I have split the high-level conversion to_plot_mesh into smaller functions to_boundary_grid, to_mesh, and to_simplex_grid. These lower-level functions do only one thing. In the previous code, some functions did several things, which was less modular.
• With these lower-level functions, the high-level conversion is much more readable than before:

function to_plot_mesh(grid::UnstructuredGrid)
  if num_cell_dims(grid) == 3
    grid |> to_boundary_grid |> to_simplex_grid |> to_mesh |> GeometryBasics.normal_mesh
  else
    grid |> to_simplex_grid |> to_mesh
  end
end

[fverdugo]

This compleates all tasks in #16.

[fverdugo]

Merged!

@paurierap I have deleted the paur-task1 branch.

Plase create a new branch paur-task2 from gsoc2021-master to start working on the task described here #17