Default `isapprox()` definition differs from Base julia

The `isapprox()` definition in this package does not match that of Julia's, causing differences when units are applied versus when they are not:

```
x = 1.0u"s"
y = 1.0u"s" + 1e-4u"s"

@show isapprox(x, y; rtol=1e-4)
@show isapprox(ustrip(x), y; rtol=1e-4)
@show isapprox(x, ustrip(y); rtol=1e-4)
@show isapprox(ustrip(x), ustrip(y); rtol=1e-4)
```

Yields:

```
isapprox(x, y; rtol = 0.0001) = true
isapprox(ustrip(x), y; rtol = 0.0001) = false
isapprox(x, ustrip(y); rtol = 0.0001) = false
isapprox(ustrip(x), ustrip(y); rtol = 0.0001) = true
```