ENH: Bring the `nan<x>` signatures up to parity with their `<x>`-based counterparts
ENH: Bring the `nan<x>` signatures up to parity with their `<x>`-based counterparts

* nanmin: initial & where
* nanmax: initial & where
* nanargmin: keepdims & out
* nanargmax: keepdims & out
* nansum: initial & where
* nanprod: initial & where
* nanmean: where
* nanvar: where
* nanstd: where

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