1. Use references to avoid copies where possible as per David's
suggestions
2. Specialized operator[] for IndividualFieldValue<string> and
SharedField<string> so that if the original VCF field type is of type
String, the index argument for operator [] cannot be greater than 0. If
it is greater than 0, then an out_of_range exception is thrown.
3. Added test case line to throw and catch the above exception.