Describe the bug
A clear and concise description of what the bug is.
Row sum needs to be divided by number of columns instead of rows

To Reproduce
Steps to reproduce the behavior:

let mat = matrix
                    [
                        [20.;  11.];
                        [6.;  29.];
                        [12.;  8.];
                    ]
Matrix.meanRowWise mat

Expected behavior
Vector [15.5; 17.5; 10]

Actual Behavior
Vector [10.33333333; 11.66666667; 6.666666667]