Before, you could have this code
```
A = SparseMatrix(2, 2, [0, 2, 3], [0], [2])
B = DenseMatrix(2, 2, [2, 0, 0, 0])
B == A # False
A == B # True
```
The second would be `True` as `SparseMatrix` already checks for semantic
equality. This commit changes `DenseMatrix` so that equality is
semantical as well.
## What changes were proposed in this pull request?
Better semantic equality for DenseMatrix
## How was this patch tested?
Unit tests were added, plus manual testing. Note that the code falls back to the old behavior when `other` is not a SparseMatrix.
Closes#17968 from gglanzani/SPARK-9792.
Authored-by: Giovanni Lanzani <giovanni@lanzani.nl>
Signed-off-by: Holden Karau <holden@pigscanfly.ca>
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