new a-grid PDFs, add decluster method tree (equal 1/3 weights) for usgs grids

stable-crust/grid/grid-data/pdf-gk-adaptive.csv

stable-crust/grid/grid-data/pdf-gk-fixed.csv

stable-crust/grid/grid-data/pdf-nn-adaptive.csv

stable-crust/grid/grid-data/pdf-nn-fixed.csv

stable-crust/grid/grid-data/pdf-r85-adaptive.csv

stable-crust/grid/grid-data/pdf-r85-fixed.csv

stable-crust/grid/usgs/gk/adaptive/rate-tree.json

stable-crust/grid/usgs/gk/adaptive/rupture-sets.json

stable-crust/grid/usgs/gk/fixed/rate-tree.json

stable-crust/grid/usgs/gk/fixed/rupture-sets.json

stable-crust/grid/usgs/gk/source-tree.json

stable-crust/grid/usgs/nn/adaptive/rate-tree.json

stable-crust/grid/usgs/nn/adaptive/rupture-sets.json

stable-crust/grid/usgs/nn/fixed/rate-tree.json

stable-crust/grid/usgs/nn/fixed/rupture-sets.json

stable-crust/grid/usgs/nn/source-tree.json

stable-crust/grid/usgs/r85/adaptive/rate-tree.json

stable-crust/grid/usgs/r85/adaptive/rupture-sets.json

stable-crust/grid/usgs/r85/fixed/rate-tree.json

stable-crust/grid/usgs/r85/fixed/rupture-sets.json