func (v *VecDense) At(i, j int) float64 {
	if uint(i) >= uint(v.n) {
		panic(ErrRowAccess)
	}
	if j != 0 {
		panic(ErrColAccess)
	}
	return v.at(i)
}
At returns the element at row i.
It panics if i is out of bounds or if j is not zero.
package mat
func (m *Dense) At(i, j int) float64 {
	if uint(i) >= uint(m.mat.Rows) {
		panic(ErrRowAccess)
	}
	if uint(j) >= uint(m.mat.Cols) {
		panic(ErrColAccess)
	}
	return m.at(i, j)
}
func (m *Dense) at(i, j int) float64 {
	return m.mat.Data[i*m.mat.Stride+j]
}
func (m *Dense) Set(i, j int, v float64) {
	if uint(i) >= uint(m.mat.Rows) {
		panic(ErrRowAccess)
	}
	if uint(j) >= uint(m.mat.Cols) {
		panic(ErrColAccess)
	}
	m.set(i, j, v)
}
func (m *Dense) set(i, j int, v float64) {
	m.mat.Data[i*m.mat.Stride+j] = v
}