[SQL][API] ComputableColumn vs IncomputableColumn

This patch changes Column from a concrete implementation to a trait, and provides two
concrete implementations: IncomputableColumn and ComputableColumn.

sql/core/src/main/scala/org/apache/spark/sql/ComputableColumn.scala

@@ -0,0 +1,33 @@
+/*
+* Licensed to the Apache Software Foundation (ASF) under one or more
+* contributor license agreements.  See the NOTICE file distributed with
+* this work for additional information regarding copyright ownership.
+* The ASF licenses this file to You under the Apache License, Version 2.0
+* (the "License"); you may not use this file except in compliance with
+* the License.  You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+package org.apache.spark.sql
+
+import scala.language.implicitConversions
+
+import org.apache.spark.sql.catalyst.expressions.Expression
+import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
+
+
+private[sql] class ComputableColumn protected[sql](
+    sqlContext: SQLContext,
+    protected[sql] val plan: LogicalPlan,
+    protected[sql] val expr: Expression)
+  extends DataFrameImpl(sqlContext, plan) with Column  {
+
+  override def isComputable: Boolean = true
+}