Make mismatch and deletes positions accessible

Follow on from #329 - this is to be able to answer a question like "How many mismatches are with k bases of each other?"  I'd only need the mismatch positions for this, but I could imagine someone wanting to additionally query "How many mismatches are with k bases of each other and are they the same change?"

And very perhaps similarly with deletions, so I removed private from all of them, but we can do it piecemeal