Plots enhancing

[didi-hou]

This PR enhanced multiple plots in the jupyter notebook, including the way of computing statistics, figure titles, color schemes, range of color bars, and so on.

• Changed the figure title in plot 2.4
• Replaced the normalized firing rate to original firing rate for visualization in plot 3.2
• Removed the normalization and limit of the color bar in plot 3.2
• Changed the color scheme used in plot 3.3
• Removed the normalization and limit of the color bar in plot 3.3
• Added annotation texts of X for the cells of population 4E and 4I of area TH