code: public Matrix insert(Matrix that) {
          return insert(that, 0, 0, 0, 0, that.rows(), that.columns());
      }
docs: Inserts a given {@code that} (B) into this matrix (A). The original
  values are overwritten by the new ones.
  
  @param that the matrix to insert, from the first row and column
  @return a matrix with the parameter inserted into it