Fix zero division error

[nmendozam]

If only one allele was genotyped for a gene, there is a zero division when calculating l95 and u95. This pull request solves that issue.

How to replicate the error:

First remove all alleles but one from a random gene in a data set, then run the following command.

python PyHLA/PyHLA.py --input input.pyhla --assoc --print

Error message

Traceback (most recent call last):
  File "PyHLA/PyHLA.py", line 2995, in <module>
    assoc = assocADRChiFisher(INFILE, DIGIT, FREQ, TEST,MODEL,ADJUST,EXCLUDE, PERM, SEED)
  File "PyHLA/PyHLA.py", line 1122, in assocADRChiFisher
    se = math.sqrt(1.0/n1  + 1.0/n2 +  1.0/n3 + 1.0/n4)
ZeroDivisionError: float division by zero