Error on mixed vector vars for Silo I/O and matset field conversions

[JustinPrivitera]

I need to error on the mixed vector var case:

field:
   topology: "topo"
   association: "element"
   values:
      a: [1,2,5,4,4,5,8,6]
      b: [1,2,5,4,4,5,8,6]
      c: [1,2,5,4,4,5,8,6]
   matset: "mset"
   matset_values:
      a: 
         mat1: [1,1,234,32,4545,...]
         mat2: [1,1,234,32,4545,...]
      b:
         mat1: [1,1,234,32,4545,...]
         mat2: [1,1,234,32,4545,...]
      c:
         mat1: [1,1,234,32,4545,...]
         mat2: [1,1,234,32,4545,...]

OR

field:
   topology: "topo"
   association: "element"
   values:
      a: [1,2,5,4,4,5,8,6]
      b: [1,2,5,4,4,5,8,6]
      c: [1,2,5,4,4,5,8,6]
   matset: "mset"
   matset_values:
      mat1:
         a: [1,1,234,32,4545,...]
         b: [1,1,234,32,4545,...]
         c: [1,1,234,32,4545,...]
      mat2: 
         a: [1,1,234,32,4545,...]
         b: [1,1,234,32,4545,...]
         c: [1,1,234,32,4545,...]

This case is very uncommon; but we should catch it and error when writing to Silo and when doing matset field conversions between the representations and when converting to_silo.