Sam: add insert_size member function

fixes #224
If it was your intention to no longer test this here (which I'm not convinced is a good idea), you should have stated that explicitly in your commit, and refactored the tests appropriately. As it stands now, we have 2 duplicate tests, one called sam_body_simple_members_by_copy and one called sam_body_simple_members_by_reference, with the exact same code, which no longer test what their names imply (copy vs. reference access)