add NAttrs and NVars methods
rename Open to OpenFile

gofmt -w -r 'Open -> OpenFile' .
rename File to Dataset

so when we support Groups in the future, they won't be called "File".
Dataset.AddGroup() and Dataset.ParentGroup() would return a Dataset.
"Dataset" also matches the terminology used by the NetCDF C library.
rename Var.f to Var.ds
be consistent about tagged/untagged struct literal