[SPARK-5486] Added validate method to BlockMatrix

The `validate` method will allow users to debug their `BlockMatrix`, if operations like `add` or `multiply` return unexpected results. It checks the following properties in a `BlockMatrix`:
- Are the dimensions of the `BlockMatrix` consistent with what the user entered: (`nRows`, `nCols`)
- Are the dimensions of each `MatrixBlock` consistent with what the user entered: (`rowsPerBlock`, `colsPerBlock`)
- Are there blocks with duplicate indices

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #4279 from brkyvz/SPARK-5486 and squashes the following commits:

c152a73 [Burak Yavuz] addressed code review v2
598c583 [Burak Yavuz] merged master
b55ac5c [Burak Yavuz] addressed code review v1
25f083b [Burak Yavuz] simplify implementation
0aa519a [Burak Yavuz] [SPARK-5486] Added validate method to BlockMatrix