Update sitar package & add new growth references

[tamuri]

sitar package updated to 1.1.2 and the following growth references now shown:

UK 1990 growth reference UK-WHO growth reference including preterm UK-WHO growth reference omitting preterm CDC growth reference 2000 WHO 2006 growth standard WHO 2006 growth standard & WHO 2007 growth reference IOTF international body mass index reference

[tamuri]

Also took the opportunity to rename tabs, but more work to do on this (i.e. adding the new headers on each tab)

[statist7]

I've checked all the growth references and they look good. A few comments:

• Some curves have duplicated ages, e.g. UK-WHO height and BMI at age 2 and 4 years, indicating that two distinct references meet at that age. Ideally they should plot both values at the duplicated age, so the line segment is vertical. But in practice just one of the two values is stored and plotted. Not sure how to handle this.
• The age range for each reference is displayed. However this is not necessarily the age range for the current measurement, so there are missing data at the start of he range. Would it make sense for the age range to apply to the currently selected measurement?
• The CDC growth reference 2000 should be called the CDC 2000 growth reference.
• The IOTF reference is for BMI, but it doesn't work with the sample dataset as there is no BMI column. There needs to be a way to say, calculate BMI from weight and height. And ideally it would use a user-specified formula that could be extended to other constructed measures such as sitting height divided by leg length.
• I generated two errors: Warning in force(expr) input string '?' cannot be translated to UTF-8, is it valid in US-ASCII? Unfortunately I don't know what I did to generate them...

[tamuri]

Thanks, Tim. I've added your comments as comments to existing issues so that can be fixed in different PR.