Address review comments.
Address review comments.

Add to docstrings. Add doctests examples for degenerates, and use
numpy to vectorize gc_frequency

SamStudio8/scikit-bio,wdwvt1/scikit-bio,gregcaporaso/scikit-bio,wdwvt1/scikit-bio,kdmurray91/scikit-bio,colinbrislawn/scikit-bio,corburn/scikit-bio,xguse/scikit-bio,SamStudio8/scikit-bio,colinbrislawn/scikit-bio,averagehat/scikit-bio,anderspitman/scikit-bio,jdrudolph/scikit-bio,demis001/scikit-bio,jairideout/scikit-bio,demis001/scikit-bio,xguse/scikit-bio,jairideout/scikit-bio,gregcaporaso/scikit-bio,corburn/scikit-bio,kdmurray91/scikit-bio,jdrudolph/scikit-bio,averagehat/scikit-bio,anderspitman/scikit-bio