camera improvements for 2d use cases

[cyrush]

Camera info does not show 2d mode params and we should add an explicit 2d mode camera.

If you have a 2d dataset with no camera specified, you get a 2D style camera.

However, all camera params we currently accept are 3d, if you provide any camera you are back in 3D mode.

[cyrush]

3d Camera:

    camera: 
      position: [-8.88178419700125e-16, -8.88178419700125e-16, 28.2842712402344]
      look_at: [-8.88178419700125e-16, -8.88178419700125e-16, 0.0]
      up: [0.0, 1.0, 0.0]
      zoom: 1.0
      fov: 60.0
      near_plane: 2.82842707633972
      far_plane: 282.842712402344

This info is missing xpan, ypan (note: zoom is effectively zpan)
We could also add the projection and view matrix info, but we can't directly set those in VTK-m.

Proposed 2d Camera Input Spec:

  camera: 
    2d: [l,r,b,t]

or more verbose:

  camera: 
    2d:
       left: l
       right: r
       bottom: b
       top: t

[cyrush]

#1484 adds a 2d camera mode for rendering and scalar rendering (project_2d), as well as 2d cam details to info along with xpan and ypan.

[cyrush]

resolved in #1484