ENH: improve gltf handling

- scene

  - write with fileName, additional getMesh accessor

  - addColourToMesh accepts an alpha field size 1 as a constant
    alpha value

  - sceneWriter wrapper

ENH: improve gltf handling of colour and alpha specification

- accept plain input directly.
  Eg,
      colour  (1 0 1);
  vs
      colour      uniform;
      colourValue (1 0 1);

- use field magnitude for colouring of non-scalar fields.

  Eg, having three different colour maps for a vector field simply
  does not help much with visualisation.