exon reduction should expand to W for 2 field alleles

[pbashyal-nmdp]

When reducing A*01:01:01:03, py-ard reduces correctly to 3 fields:

>>> ard.redux('A*01:01:01:03', 'exon')
'A*01:01:01'

When reducing an allele at ARD level, eg: A*01:01, it currently produces:

>>> ard.redux('A*01:01', 'exon')
'A*01:01'

To correctly reduce to the exon version, it should first expand to W and then reduce to exon level producing all possibilities at exon level.
e.g.

>>> ard.redux(ard.redux('A*01:01', 'W'), 'exon')
'A*01:01:01N/A*01:01:01/A*01:01:02/A*01:01:03/A*01:01:04/A*01:01:05/A*01:01:06/A*01:01:07/A*01:01:08/A*01:01:09/A*01:01:10/A*01:01:11/A*01:01:12/A*01:01:13/A*01:01:14/A*01:01:15/A*01:01:16/A*01:01:17/A*01:01:18/A*01:01:19/A*01:01:20/A*01:01:21/A*01:01:22/A*01:01:23/A*01:01:24/A*01:01:25/A*01:01:26/A*01:01:27/A*01:01:28/A*01:01:29/A*01:01:30/A*01:01:31/A*01:01:32/A*01:01:33/A*01:01:34/A*01:01:35/A*01:01:36/A*01:01:37/A*01:01:38L/A*01:01:39/A*01:01:40/A*01:01:41/A*01:01:42/A*01:01:43/A*01:01:44/A*01:01:45/A*01:01:46/A*01:01:47/A*01:01:48/A*01:01:49/A*01:01:50/A*01:01:51/A*01:01:52/A*01:01:53/A*01:01:54/A*01:01:55/A*01:01:56/A*01:01:57/A*01:01:58/A*01:01:59/A*01:01:60/A*01:01:61/A*01:01:62/A*01:01:63/A*01:01:64/A*01:01:65/A*01:01:66/A*01:01:67/A*01:01:68/A*01:01:69/A*01:01:70/A*01:01:71/A*01:01:72/A*01:01:73/A*01:01:74/A*01:01:75/A*01:01:76/A*01:01:77/A*01:01:78/A*01:01:79/A*01:01:80/A*01:01:81/A*01:01:82/A*01:01:83/A*01:01:84/A*01:01:85/A*01:01:86/A*01:01:87/A*01:01:88/A*01:01:89/A*01:01:90/A*01:01:91/A*01:01:92/A*01:01:93/A*01:01:94/A*01:01:95/A*01:01:96/A*01:01:97/A*01:01:98/A*01:01:99/A*01:01:100/A*01:01:101/A*01:01:102/A*01:01:103/A*01:01:104/A*01:01:105/A*01:01:106/A*01:01:107/A*01:01:108/A*01:01:109/A*01:01:110/A*01:01:111/A*01:01:112/A*01:01:113/A*01:01:114/A*01:01:115/A*01:01:116/A*01:01:117/A*01:01:118/A*01:01:119/A*01:01:120/A*01:01:121/A*01:01:122/A*01:01:123/A*01:01:124/A*01:01:125/A*01:01:126/A*01:01:127/A*01:01:128/A*01:01:129/A*01:01:130/A*01:01:131/A*01:01:132/A*01:01:133/A*01:01:134/A*01:01:135/A*01:01:136/A*01:01:137/A*01:01:138/A*01:01:139/A*01:01:140/A*01:01:141/A*01:01:142/A*01:01:143/A*01:01:144/A*01:01:145/A*01:01:146/A*01:01:147/A*01:01:148/A*01:01:149/A*01:01:150/A*01:01:151/A*01:01:152/A*01:01:153'