IndividualFieldValueIterator: Fix misplaced const qualifiers

Because IndividualFieldValueIterator implements operator=, declaring
members as const should cause the compiler to throw errors on the
assignment operator implementation.

NVIDIA's nvcc compiler front-end signals this as a problem, even though
gcc does not. It's unclear whether this can lead to undefined behavior.

Remove the const qualifiers from IndividualFieldValueIterator member
variables.