MTH: Update quantile default lerp method
MTH: Update quantile default lerp method

For method 7 of H&F, using `(n - 1) * quantiles`
instead of the usual method gives a more accurate
result.

pdebuyl/numpy,seberg/numpy,anntzer/numpy,pdebuyl/numpy,anntzer/numpy,seberg/numpy,numpy/numpy,endolith/numpy,jakirkham/numpy,jakirkham/numpy,rgommers/numpy,mhvk/numpy,mhvk/numpy,seberg/numpy,rgommers/numpy,endolith/numpy,seberg/numpy,rgommers/numpy,pdebuyl/numpy,anntzer/numpy,mhvk/numpy,pdebuyl/numpy,mattip/numpy,mhvk/numpy,mattip/numpy,jakirkham/numpy,mhvk/numpy,charris/numpy,rgommers/numpy,numpy/numpy,charris/numpy,charris/numpy,mattip/numpy,jakirkham/numpy,jakirkham/numpy,mattip/numpy,numpy/numpy,endolith/numpy,numpy/numpy,anntzer/numpy,charris/numpy,endolith/numpy