Switch file format from '.feather' to '.arrow'

[bmschmidt]

I've used '.feather' as the file format here, but over the last couple years the community has standardized around '.arrow' instead, to the point where the mailing list is now discussing deprecating the 'feather' methods entirely and reminding me that ".arrow is the official registered extension."

This will only get worse as time goes on, so will be changed in the next version bump. This is going to cause some breakage and will probably need an ability to specify, in plots, use of the '.feather' extension.