IndividualFieldValue: Fix misplaced const qualifiers

The implementation of IndividualFieldValue::operator= assigns new
values to member variables declared as const, which is not legal.
NVIDIA's nvcc compiler front-end signals this as a problem, even though
gcc sometimes accepts it.

Remove the const qualifiers from IndividualFieldValue member variables.
Apply same fix to IndividualFieldValueIterator members as well.
Merge pull request #248 from nsubtil/const-assignments

IndividualFieldValue: Fix misplaced const qualifiers