ce_plot()

• add wtp_linetype argument to ce_plot() (and default to "dashed") to given control to user and differentiate C-E plane from main axes.
• add shape argument which maps to the shape aesthetic in geom_point(). User can also specify shape = "cost-effective" to map it to whether that point is under the WTP/cost-effectiveness plane, or shape = "method" to map it to the cutpoint method.
• add add_prop_ce argument. If TRUE, the % of points for that cutpoint method is concatenated to the the method name in the figure legend.
• outcome costs were incorrectly excluded from the costs used in the incremental cost axis before. These are now added to the calculations in evaluate_cutpoint_cost() and reflected in the ce_plot(). Before, the incremental costs in ce_plot() were treatment costs alone.

Track QALYs and intervention costs

• Track QALYs and intervention costs separately from NMB within simulations
• get_nmb_sampler() now returns a NMBsampler object with attributes to indicate whether QALYs should be tracked during simulations and the WTP used
• If QALYs are tracked, ce_plot() can be used to create a cost-effectiveness plot

Updates to README, vignettes and documentation for clarity

Many updates in response to peer review with ropensci see #566 (response comment)

• Efficiency improvements and generally improved coding style
• Use of autoplot() method instead of plot()
• Use of summary() method instead of make_summary_table()
• Removal of default themes in plots in exchange for ... + theme_sim()
• Improvements and corrections to vignettes