Fixed IndividualFieldIterator assignment operator

The format pointer was a const member, which resulted in compiler errors when trying to use operator=.  There was an additional bug in how operator= checked for self-assignment -- if (&this == other) instead of if( this == &other). I simply removed the self-assignment check as it it not needed here and branching is not worth getting a small performance boost in the very rare case of self-assigning an iterator.