Add matrix multiplication with double[][] and unit tests (#6417)

* MatrixMultiplication.java created and updated.

* Add necessary comment to MatrixMultiplication.java

* Create MatrixMultiplicationTest.java

* method for 2 by 2 matrix multiplication is created

* Use assertMatrixEquals(), otherwise there can be error due to floating point arithmetic errors

* assertMatrixEquals method created and updated

* method created for 3by2 matrix multiply with 2by1 matrix

* method created for null matrix multiplication

* method for test matrix dimension error

* method for test empty matrix input

* testMultiply3by2and2by1 test case updated

* Check for empty matrices part updated

* Updated Unit test coverage

* files updated

* clean the code

* clean the code

* Updated files with google-java-format

* Updated files

* Updated files

* Updated files

* Updated files

* Add reference links and complexities

* Add test cases for 1by1 matrix and non-rectangular matrix

* Add reference links and complexities

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Co-authored-by: Deniz Altunkapan <93663085+DenizAltunkapan@users.noreply.github.com>