IndividualFieldValue: Fix misplaced const qualifiers

Because IndividualFieldValue 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 IndividualFieldValue member variables.
Apply same fix to IndividualFieldValueIterator members as well.