add a groupby method (#64)

Author: rafaqz

Project.toml

@@ -1,7 +1,7 @@
 name = "DataAPI"
 uuid = "9a962f9c-6df0-11e9-0e5d-c546b8b5ee8a"
 authors = ["quinnj <[email protected]>"]
-version = "1.15.0"
+version = "1.16.0"
 
 [compat]
 julia = "1"

src/DataAPI.jl

@@ -522,4 +522,8 @@ Return the row number of `row` in the source table.
 """
 function rownumber end
 
+# To avoid type piracy and method ambiguities, implementations of `groupby` 
+# must restrict the first argument to a type defined in the same package. 
+function groupby end
+
 end # module