Add 2d-density mz/rt map to QualityControlBasic

[David JM]

something like this

ggplot(m_m, aes(rt, mz)) +
  stat_density2d(aes(fill = ..density..), contour = F, geom = 'tile') +
  theme_light() +
  scale_fill_viridis() +
  ggtitle("RT/MZ")