ENH: basic i-j-k Field container (#1216)

- this is a simple container for fields with i-j-k addressing.

  It does not support field operations directly, but is primarily
  intended to be used when assembling field information with i-j-k
  logic. After assembly, the field can be transferred to a regular
  field for normal operations.  Eg,

      IjkField<scalar> assemble({15, 16, 200});

      // .. fill in i-j-k fields

      Field<scalar> final(std::move(assemble));
      assemble.clear();   // be pedantic
      ...