ut_compare fails for an identity unit

[Enchufa2]

Maybe related to #346, but a bit different: there are issues too when the scaling factor is equal to 1. More specifically:

units:::ud_compare(0.5, c(0.9, 1, 2), "m", "m")
#> [1] -1  1 -1

0.5 m is considered greater than 1 m (!).

[Enchufa2]

So it seems that ut_compare doesn't work as we expected, so we need to rework this.