`[<-` for unitted_data.frames

[appling]

Some assignments to new columns work, others don't:

> library(unitted)
> df <- u(data.frame(x=1:3, y=4:6), c('a','b'))
> df$c <- TRUE
> df['d'] <- FALSE
Error in unitted(object, units, ...) : 
  Number of units must equal number of data.frame columns
> df[,'d'] <- FALSE
Error in unitted(object, units, ...) : 
  Number of units must equal number of data.frame columns
> df[['d']] <- TRUE