implement dimensionless units becoming numbers automatically (introduces 2 new options)

[sebkopf]

a couple aspects to this:

• always simplify to unitless after multiplication/division if it's possible (i.e. the "1" should not need to be listed in default units (it's only confusing)) but make that an option that's something along the lines of simplify_unitless = TRUE(default)/FALSE and then to turn it into normal numeric declass_unitless = TRUE(default)/FALSE)
• then set_cu(1, "mg") / set_cu(1, "ng") would just become 1e6 (no units)