Documentation needed for get_fisher_bias

[elisachisari]

Doc strings needed for the function get_fisher_bias:

augur/augur/analyze.py

Line 530 in 1cbcb12

def get_fisher_bias(self, force=False, method='5pt_stencil', save_txt=True, use_fid=False):

(Some comments already available.)

[fjaviersanchez]

Should we close this by #110, @elisachisari?