automatically set level values based on data if not provided

[kkiesling]

Right now, there is the requirement to supply information about the levels because there is no way to read the data from the mesh file and determine it in a different way. With the introduction of meshio, this could be done. A user would still likely be better supplying their own values. But with meshio, the default could be to do 10 linearly spaced levels from the min to max value if there is no other levels provided.