Merge incremental plot cells

[kangwonlee]

• Some cells incrementally build up the plots; plt.show() would finalize the plot
• (Since when?) Now each cell present a plot and start anew in the next cell
• [ ] Ch02
• [x] Ch03 cffe18fcbf52746a8f5d81bad6aa7fe1f6d652e6
• [x] ex04.002 c7368cd
• [x] ex04.003
• [x] ex04.005
• [x] ex04.006
• [x] Ch05 875a37d27b28f465cb2f4fb4f66b55f0bac26431
• [x] Ch06 9fc9850
• [x] Ch07
• [x] Ch08
• [x] Ch10
• [x] Ch12