New interfaces to add or update bam integer, float and array aux tags (#694)

* Pull repeated code to expand bam data to its own function

Adds some missing overflow checks and fixes a few places where
l_data was incremented before trying to expand the data buffer
so it would no longer be valid on failure.

* Add bam_aux_update_int() interface

Makes adding or changing the values of integer tags much easier.
Updated tags will grow in size if needed, including moving
any following data.  They will not shrink - if the new data fits
in the old space the size will remain unchanged even if it is
bigger than stricly necessary.

* Add bam_aux_update_float() interface

* Add bam_aux_update_array() interface