Material-Dependent Fields:
fields/field/association: “vertex” | “element”
fields/field/basis: (mfem-style finite element collection name) (replaces “association”)
fields/field/volume_dependent: “true” | “false”
fields/field/matset: “matset”
fields/field/matset_values: (mcarray)
Missing values needs to be supported on writing and reading for silo, and for doing conversions.