Add dataset introductions to lectures #312

Job	Run time
Load user automation choices	19s
Render preview	2m 20s
Style code	0s
Check spelling / status-update	4s
Check spelling / error-check	1m 11s
Check URLs / error-check	1m 17s
Check URLs / status-update	6s
Render pdf lecture slides (Data_Classes)	42s
Render pdf lecture slides (Data_Cleaning)	44s
Render pdf lecture slides (Data_Input)	51s
Render pdf lecture slides (Data_Output)	40s
Render pdf lecture slides (Data_Summarization)	49s
Render pdf lecture slides (Data_Visualization)	53s
Render pdf lecture slides (Esquisse_Data_Visualization)	41s
Render pdf lecture slides (Factors)	46s
Render pdf lecture slides (Functions)	46s
Render pdf lecture slides (Manipulating_Data_in_R)	42s
Render pdf lecture slides (Statistics)	43s
Render pdf lecture slides (Subsetting_Data_in_R)	55s
	14m 29s

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