[Spark-6693][MLlib]add tostring with max lines and width for matrix

jira: https://issues.apache.org/jira/browse/SPARK-6693

It's kind of annoying when debugging and found you cannot print out the matrix as you want.

original toString of Matrix only print like following,
0.17810102596909183    0.5616906241468385    ... (10 total)
0.9692861997823815     0.015558159784155756  ...
0.8513015122819192     0.031523763918528847  ...
0.5396875653953941     0.3267864552779176    ...

The   def toString(maxLines : Int, maxWidth : Int) is useful when debuging, logging and saving matrix to files.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes apache#5344 from hhbyyh/addToString and squashes the following commits:

19a6836 [Yuhao Yang] remove extra line
6314b21 [Yuhao Yang] add exclude
736c324 [Yuhao Yang] add ut and exclude
420da39 [Yuhao Yang] Merge remote-tracking branch 'upstream/master' into addToString
c22f352 [Yuhao Yang] style change
64a9e0f [Yuhao Yang] add specific to string to matrix

mllib/src/main/scala/org/apache/spark/mllib/linalg/Matrices.scala

@@ -87,6 +87,9 @@ sealed trait Matrix extends Serializable {
   /** A human readable representation of the matrix */
   override def toString: String = toBreeze.toString()
 
+  /** A human readable representation of the matrix with maximum lines and width */
+  def toString(maxLines: Int, maxLineWidth: Int): String = toBreeze.toString(maxLines, maxLineWidth)
+
   /** Map the values of this matrix using a function. Generates a new matrix. Performs the
     * function on only the backing array. For example, an operation such as addition or
     * subtraction will only be performed on the non-zero values in a `SparseMatrix`. */

mllib/src/test/scala/org/apache/spark/mllib/linalg/MatricesSuite.scala

@@ -439,4 +439,20 @@ class MatricesSuite extends FunSuite {
     assert(mUDT.typeName == "matrix")
     assert(mUDT.simpleString == "matrix")
   }
+
+  test("toString") {
+    val empty = Matrices.ones(0, 0)
+    empty.toString(0, 0)
+
+    val mat = Matrices.rand(5, 10, new Random())
+    mat.toString(-1, -5)
+    mat.toString(0, 0)
+    mat.toString(Int.MinValue, Int.MinValue)
+    mat.toString(Int.MaxValue, Int.MaxValue)
+    var lines = mat.toString(6, 50).lines.toArray
+    assert(lines.size == 5 && lines.forall(_.size <= 50))
+
+    lines = mat.toString(5, 100).lines.toArray
+    assert(lines.size == 5 && lines.forall(_.size <= 100))
+  }
 }

project/MimaExcludes.scala

@@ -64,6 +64,10 @@ object MimaExcludes {
             // SPARK-6492 Fix deadlock in SparkContext.stop()
             ProblemFilters.exclude[MissingMethodProblem]("org.apache.spark.SparkContext.org$" +
                 "apache$spark$SparkContext$$SPARK_CONTEXT_CONSTRUCTOR_LOCK")
+          )++ Seq(
+            // SPARK-6693 add tostring with max lines and width for matrix
+            ProblemFilters.exclude[MissingMethodProblem](
+              "org.apache.spark.mllib.linalg.Matrix.toString")
           )
 
         case v if v.startsWith("1.3") =>