To avoid additional clutter in the interface volFields, surfaceFields and
pointFields are now selected from a single fields selection box consistent with
the single directory with guaranteed unique names in which they are stored.
Note that when visualising the "phi" flux fields that these are extensive, the
value depends directly on the face area, so unless the mesh is uniform
interpolated continuous colour plots are not physical or informative.
Based on proposal contributed by Mattijs Janssens
b7c0646ed9