Allow specification of output coords when extruding coordinate field

Rather than always using the coordinate field in the extruded mesh,
allow the user to specify the field to write to.
Allow vfamily to be DG1 when extruding coordinates
Support hedgehog extrusion: extrusion_type="radial_hedgehog"

Allow extrusion of circle and sphere meshes into a "hedgehog-like"
P1dgxP1 coordinate field where the coordinates are extruded radially
outward in the direction of the outward cell normal.  For output
purposes (and Jacobian calculations), a normal "radially" extruded P1
coordinate field is computed simultaneously and made available in the
mesh.radial_coordinates property.
Merge pull request #377 from firedrakeproject/hedgehog-extrusion

* hedgehog-extrusion:
  Support hedgehog extrusion: extrusion_type="radial_hedgehog"
  Allow vfamily to be DG1 when extruding coordinates
  Allow specification of output coords when extruding coordinate field