making python Alignment objects subscriptable

[diekhans]

Being able to index as an array as well as iterate would be useful

>>> type(blk)                                                                                              
<class 'taffy.lib.Alignment'>                                                                              
>>> blk[0]                                                                                                 
Traceback (most recent call last):                                                                         
  File "<stdin>", line 1, in <module>                                                                      
TypeError: 'Alignment' object is not subscriptable