Convert matrix from gonum.mat.Matrix to PNG picture.
Non-zero matrix element is black.
Example:
m := mat.NewDense(100, 80, nil)
for i := 0; i < 80; i++ {
m.Set(i, i, 1.0)
}
err := matpi.Convert(m, "result.png", matpi.NewConfig())
if err != nil {
return
}