Default to multiple_iterators=False for pileups
Default to multiple_iterators=False for pileups

For CRAM files, HTSlib iterators are in fact a stub struct -- the real
iterator limits are stored within the cram_fd itself, having been set
via cram_set_option(CRAM_OPT_RANGE). This means that reopening the CRAM
file as a new cram_fd, which happens with multiple_iterators=True, is
problematic as the iterator limits don't apply to the separate cram_fd.

For pileups we typically don't need multiple iterators. Fixes #725.

pysam-developers/pysam,pysam-developers/pysam,pysam-developers/pysam,pysam-developers/pysam