Fix comparisons to numbers and among units of different types

Prior to this,
```
julia> 1m<2kg
true
```
Also, since currently type inference yields
```
r = 1.0m./[1m,2m]
julia> typeof(r[1])
SIQuantity{Float64,0,0,0,0,0,0,0} (constructor with 1 method)
```
(even when inside a function), it's important to have comparisons between plain numbers and objects of type `SIQuantity{Float64,0,0,0,0,0,0,0}`.