Support mixed type elements

[brey]

This functionality is already available in pyposeidon. We need to transfer it here.

[wzhengui]

Hi George: I already fixed this in my recent PR.

[pmav99]

@brey do you mean visualizing quads? What we currently have is splitting quads in triangles. Is this good enough? If yes we should close. We can keep the ticket open to tackle this in the future though, too.

[brey]

Indeed the issue is with the visualisation of the quads. I have a solution using gv.Path but it's slow and create issues with the api. I suggest we leave it open and revisit if time permits.