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 way htslib is organized is that for a string field, the num_bytes_per_sample is equal to the space allocated for the full string. Thus, any iterator would straightaway jump to the end of the string with a single ++ operation. There would be no need to check vector_end.
In other words, the iterator does not iterate over chars, but iterates over the full string.