Feplot

- Fixing missing icons folder after publishing.

- Switching to pyvista for more plotting flexibility.
- Adding XDMFReader to read FElupe xdmf outputs.

Added:
- add 1d mesh plot command `line_plot`.
- add support for `skfem` package results.

Changed:
- Rename `polyplot` to `volume_plot` and `quatplot` to `surface_plot`.
- Minimize `plot2d` code by removing `Delaunay` Triangulation and `tricontourf` lines and pass values directly to `surface_plot` (the 2d plot is more faster now).
Added:
- Add orientation axes using the `add_axes` command.

- Fix the hexahedron mesh plot
- Improve polyplot tool speed (full numpy speed)
- Add ruler tool

* Add plot_model helper
* Enhance polyplot tool