remove options. Always parse units?

clean up mess of as.character, maybe split into as.label?

address units "100 km" -> "100 * km" in label

address inch: 

inch <- make_unit("in"); still, auto plotting "in" breaks axis labels:

> library(units)
> inch = make_unit("in")
> x = rnorm(100) * inch
> hist(x)
Error in parse(text = str) : <text>:1:14: unexpected 'in'
1: x*~group('[',in
                 ^

