Add index number to incrementals

[spolcyn]

It would be good if incrementals internally kept track their index number so trying to append the same incremental multiple times would be an error (in case for example a network message arrived twice). Currently, a BidsRun is an append-only data structure.

To Change:

1. BidsIncremental: Add index number as (required?) field.
2. BidsRun: Verify appends are in order.