This function returns a matrix of component variables, X. The user can specify
the desired chemicals and order by creating a list of string vectors, each vector containing the variable names of all
desired elements of that group.
Usage
make.X(df, num.groups, groups)
Arguments
df
A dataframe containing named component variables
num.groups
An integer representing the number of component groups desired
groups
A list, each item in the list being a string vector of variable names for one component group