[SYSTEMML-902] Improve FrameObject toString output

[deroneriksson]

Override toString for FrameObject, similar to MatrixObject.

Example of outputting MatrixObject and FrameObject via MLContext, before PR:

scala> val res = ml.execute(dmlScript)
res: org.apache.sysml.api.mlcontext.MLResults = 
  [1] (Matrix) tA: Matrix: scratch_space//_p7667_9.31.117.12//_t0/tA_290, [3 x 2, nnz=6, blocks (1000 x 1000)], binaryblock, dirty
  [2] (Frame) tAM: org.apache.sysml.runtime.controlprogram.caching.FrameObject@6c6c4579

After PR:

scala> val res = ml.execute(dmlScript)
res: org.apache.sysml.api.mlcontext.MLResults = 
  [1] (Matrix) tA: Matrix: scratch_space//_p42840_9.31.117.12//_t0/tA_1, [3 x 2, nnz=6, blocks (1000 x 1000)], binaryblock, dirty
  [2] (Frame) tAM: Frame: scratch_space//_p42840_9.31.117.12//_t0/tAM, [3 x 2, nnz=6, blocks (1000 x 1000)], binaryblock, dirty

[deroneriksson]

@mboehm7
Is the conditional "if (_metaData instanceof NumItemsByEachReducerMetaData)" check necessary for frames or can this be removed?

[mboehm7]

no it's not required as frames are not supported in the MR backend yet.

[deroneriksson]

@mboehm7 Thank you