Handle bcf_*_vector_end  - DO NOT MERGE YET

1. Modified SharedFieldIterator and IndividualFieldValueIterator to handle bcf_*_vector_end values. This is done by passing the end value of
the pointer in the constructor and modifying the "==" operator of the
iterator. The idea is that 2 iterators x,y are equal iff
 (a) x and y are identical  OR
 (b) (x == end && *y == bcf_*_vector_end) || (y == end && *x == bcf_*_vector_end)
2. Inserted test case fields for variable length vectors
The cached value is useful if and only if the index argument for operator[] is equal to 0. I have modified the function, but it's unclear whether it's useful