Add AlignedSegment is_mapped/mate_is_mapped/is_forward/mate_is_forward properties

[jmarshall]

These are opposites of the existing is_unmapped/mate_is_unmapped/is_reverse/mate_is_reverse properties. The existing properties match the sense of the underlying flag bits; these proposed additions allow more natural conditions to be written in some cases:

if not aln.is_unmapped:
    print('stilted way to say if mapped')

if aln.is_mapped:
    print('more natural')

if aln.is_forward:
    print('handle the forward case first')
else:
    print('then the reverse case')

Inspired by a minor grizzle by Nick Loman.

[AndreasHeger]

Thanks, it is nice to be nice :=)

[jmarshall]

Thanks!