An input field that comes from a 2D array. The first dimension
of the array will be used to read each successive row. The second
dimension is fixed, and specified in the constructor. You would create
multiple InputFieldArray2D object to read each of the "columns" stored
at each row.
Note: this input field will not be persisted to an EG file.
This is because it could point to a lengthy array, that really
has no meaning inside of an EG file.