Refactor measures shown in histogram

[jwildfire]

Let user specify which measures are drawn when calling safetyHistogram(). Allow arrays or string. Multiple pages? Panels?

[jwildfire]

I'd suggest adding settings$measure_values that takes a string ("Cholesterol") or a vector of strings (c("Cholesterol", "Diastolic Blood Pressure")) and then provides histograms for the specified measure(s).

Might also be nice to also add settings$page_layout that would let the user specify whether to show one plot per page or to use small multiples (probably via ggplot2 facet_grid). Could also cover that in a future issue though ...

[isaaczhao23]

• If measure_values is unspecified, defaults to first parameter. If one measure_value specified, prints 1 graph. If k measure_values specified, prints k graphs, 3 per row max.
• If x limits specified (low_lim, up_lim), must be in same order as the measure_values
• Couldn't get custom binwidth to work due to the nature of facet_wrap, but there seems to be potential solutions on stackoverflow
• Plot size display would probably need to be done within UI of shinyapp itself. Not sure best approach for this

settings[["measure_values"]] = c("Bicarbonate","Chloride")
settings[["low_lim"]] = c(20,90)
settings[["up_lim"]] = c(35,120)

https://github.com/SafetyGraphics/safetyCharts/blob/IZ_dev/R/safety_histogram.R