aggregate2sources: aggregates two variables from two sources into one
In grebbel/formr_test: formr survey framework

Takes two variables with different missings and gives one variable with values of the second variable substituted where the first had missings.

1 2	aggregate2sources(df, new_var, var1 = NULL, var2 = NULL, remove_old_variables = TRUE)

`df`	data.frame or variable
`new_var`	new variable name
`var1`	first source. Assumed to be new_var.x (default suffixes after merging)
`var2`	second source. Assumed to be new_var.y (default suffixes after merging)
`remove_old_variables`	Defaults to not keeping var1 and var2 in the resulting df.

cars$dist.x = cars$dist
cars$dist.y = cars$dist
cars$dist.y[2:5] = NA
cars$dist.x[10:15] = NA # sprinkle missings
cars$dist = NULL # remove old variable
cars = aggregate2sources(cars, 'dist')