Netgen: curved meshes from Mesh constructor and refine_marked_elements

[pbrubeck]

Description

Previously we had 2 methods to construct a curved Mesh out of a netgen mesh:

1. First creating a firedrake Mesh from a linear netgen mesh, and then calling curve_field(degree) to produce new coordinates, then you could create a firedrake Mesh out of the curve field. The final Firedrake Mesh will not know that it comes from a netgen mesh, and therefore cannot be adaptively refined.

mesh1 = Mesh(netgen_mesh)
meshp = Mesh(mesh1.curve_field(p))

2. Via the MeshHierarchy constructor, this method produces a MeshHierarchy in which each mesh knows that it comes from a netgen mesh

mesh1 = Mesh(netgen_mesh)
mh = MeshHierarchy(mesh1, refine, netgen_flags={order: p})
meshp = mh[0]

This PR enables the more intuitive approach (while not removing the other two)

meshp = Mesh(netgen_mesh, netgen_flags={order: p})

Also propagates the mesh order in refine_marked_elements.

[connorjward]

This is a way better way to build these meshes. Could you amend our demos?