Calculate quantiles for normal distribution in numerically stable way
Calculate quantiles for normal distribution in numerically
stable way

old method losed precision for small p. Expression (1-p) was at fault
correct way is to flip sign of erfc after computation

Fixes #64

mihaimaruseac/statistics,bos/statistics,fpco/statistics,bos/statistics,Shimuuar/statistics,Shimuuar/statistics,mihaimaruseac/statistics,fpco/statistics,mihaimaruseac/statistics