get a list of mapped reads
	require that both pairs are mapped in the sam file in order to remove the reads

def sam_list_paired(sam):
	"""
	get a list of mapped reads
	require that both pairs are mapped in the sam file in order to remove the reads
	"""
	list = []
	pair = ['1', '2']
	prev = ''
	for file in sam:
		for line in file:
			if line.startswith('@') is False:
				line = line.strip().split()
				id, map = line[0], int(line[1])
				if map != 4 and map != 8:
					read = id.rsplit('/')[0]
					if read == prev:
						list.append(read)
					prev = read
	return set(list)