GSOC-Task #4 Refactoring

[fverdugo]

This major refactoring solves these problems:

• We don't need to expose new recipe names to the user
• We solve the issue with Grid and Triangulation

[fverdugo]

• [x] Work on branch pr-task4 and abandon pr-task2.
• [x] Address the TODOs in the source code. In particular, extensible processing of attributes.
• [x] Move code in src/GridapMakie.jl to src/recipes.jl
• [x] Implement the shorthand plot(uh)
• [x] Update the tests according the new API. I.e., replace cells by mesh or plot depending the case.
• [ ] Sensible default values when using plot and mesh.

[fverdugo]

• [x] First version of README.md. Use _readme folder, Literate.jl, etc
  • Tentative static figs (ids in runtests.jl): 1, 11, 12, 111
  • Dynamic figs.
• [x] Open a PR gsoc_master -> master
• [ ] Write the project description etc in the PR description
  • See examples from last years.
  • Structure
    • Summary of the project
    • Initial state
    • What I have done
    • How I have done it (link to issues, mention slak, weekly meetings)
    • Link to results/examples (README, link to package index)
    • future work
  • Relate it with the issues we have used for the tasks
  • Relate it with the project proposal
  • Open and link issues with future work
  • quads / hexs
  • arrows
  • sections and slices
• [x] Open PR from pr_task4 to gsoc_master. Once merged, the global PR will be automatically updated.
• [x] Try to solve pending issues
  • 3D visualization of celldata and fields
  • colorrange

[fverdugo]

Fixed by PR #24