calcFatFreeMass: calcFatFreeMass
In UppsalaHenrik/pediutils: Some simple utilities for PMX modelling in paediatrics.

Calculates fat free mass for each row in a data frame and returns a vector of the same length as the number of rowss in that data frame.

calcFatFreeMass(df, idVar = "ID", ageVar = "AGE", weightVar = "WT",
  heightVar = "HT", sexVar = "SEX", ageUnit = "years",
  heightConv = 0.01, weightConv = 1, femaleSexVal = 2, maleSexVal = 1,
  childCutoff = 18, missingVal = -99)

`df`	A data frame with all necessary columns for calculation of fat free mass.
`idVar`	Name of the id column in df. Default is "ID" but it is not case sensitive.
`ageVar`	Name of the age column in df. Default is "AGE" but it is not case sensitive.
`weightVar`	Name of the weight column in df. Default is "WT" but it is not case sensitive.
`heightVar`	Name of the height column in df. Default is "HT" but it is not case sensitive.
`sexVar`	Name of the sex column in df. Default is "SEX" but it is not case sensitive.
`ageUnit`	A single string describing the unit for age in the data. "years", "months", or "days" are allowed. Defult is "years"
`heightConv`	Height conversion factor to meters. Height*heightConv is height in meters. Default is 0.01, meaning that centimeters is expected in data.
`weightConv`	Weight conversion factor to kilograms. Default is 1, meaning that kilograms is expected in data. There is currently no support for stones or pounds.
`femaleSexVal`	The value denoting female sex in the supplied df. Default is 2.
`maleSexVal`	The value denoting male sex in the supplied df. Default is 1.
`childCutoff`	The age, in years, at which a person is considered to be an adult. Default is 18.
`missingVal`	The value that will be inserted in place of Z-scores that cannot be determined. Either because age is out of bounds of the reference WHO data, or because one of the required values for age, weight, or height was missing from input data. Default is -99 according to Perl speaks NONMEM standard.