Compute aggregation function on specified numeric columns. For example:

    val df = Seq(("a", 1, 0, "b"), ("b", 2, 4, "c"), ("a", 2, 3, "d")).toDataFrame("key", "value1", "value2", "rest")
    df.groupBy("key").min("value2")

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4592 from viirya/specific_cols_agg and squashes the following commits:

9446896 [Liang-Chi Hsieh] For comments.
314c4cd [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into specific_cols_agg
353fad7 [Liang-Chi Hsieh] For python unit tests.
54ed0c4 [Liang-Chi Hsieh] Address comments.
b079e6b [Liang-Chi Hsieh] Remove duplicate codes.
55100fb [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into specific_cols_agg
880c2ac [Liang-Chi Hsieh] Fix Python style checks.
4c63a01 [Liang-Chi Hsieh] Fix pyspark.
b1a24fc [Liang-Chi Hsieh] Address comments.
2592f29 [Liang-Chi Hsieh] Merge remote-tracking branch 'upstream/master' into specific_cols_agg
27069c3 [Liang-Chi Hsieh] Combine functions and add varargs annotation.
371a3f7 [Liang-Chi Hsieh] Compute aggregation function on specified numeric columns.