Allow missing unit?

[paulponcet]

NA_unit_ could be created as follows:

NA_unit_ = structure(list(numerator = NA_character_, denominator = character(0)), 
                     class = "symbolic_units")

Operations +, -, etc. on vectors could then be updated in order to propagate a missing unit. For instance if a vector x is in meters and y has a missing unit, then x * y should have a missing unit.

[Enchufa2]

This is being tracked in #163. Please, continue the discussion there.