Creating new section for population vimp

[bdwilliamson]

Creates new section 4 for population vimp Also runs more super learners, for conditional and marginal population vimp Updates on 6 December 2019:

• change SL runs for variable importance to match with new vimp package version (requires sample splitting for hypothesis testing)
• update VIM summary in executive summary to compare ranks of variable groups across outcomes
• add a flag for running individual VIMs with SL and vimp package (defaults to FALSE)
• only print individual VIM results if the individual VIM flag is TRUE
• add sample sizes to all figure and table captions