Implement operator*(scalar, matrix) for Matrix2 and Matrix3

As remarked by @palmerb4 in #151 (closed), the operator*(scalar, matrix) for Matrix2 and Matrix3 is currently commented. According to the test for Matrix3, this was done to prevent compilation errors when multiplying two objects of Matrix3 or Matrix2, respectively.

On the basis of by @palmerb4's suggestion in #151 (closed), this merge request provides a new implementation of the operator*(scalar, matrix) that does not result in the above mentioned errors.